Since, For and Ago: Three Words That Decide Your Tense

1Time words are not decoration

Most learners treat since, for and ago as vocabulary — three words with three meanings, learn them and move on. That misses what actually makes them difficult.

These three words do not just add information to a sentence. They CONSTRAIN the tense. Choosing a time word and then choosing a tense independently is how the classic errors get made.

"I live here since 2019" is a good example. The word since is correct — 2019 is genuinely the starting point. What is wrong is that since demands a perfect tense, and live is in the present simple. The learner picked the right word and the wrong verb, and the sentence broke.

So the useful way to think about them is as instructions to the verb. Since says: use a perfect tense. Ago says: use the past simple. For says nothing, which is why it is the flexible one and the source of a different kind of error.

Fix these three: (a) I know her since 2015. (b) I have finished it two hours ago. (c) I am here for two hours.

(a) I have known her since 2015 — since demands the perfect. (b) I finished it two hours ago — ago demands the past simple. (c) I have been here for two hours — for allows both, but the meaning here is that you are still here, so the perfect is needed.

Tip: If you fix nothing else, fix since. It is the highest-frequency error of the three and it appears in the first minute of most conversations about your life.

2Since: the starting point

Since answers the question "starting when?" and it is the most rigid of the three.

since + a POINT in time. Not a duration. since 2019, since Monday, since March, since I was a child, since breakfast, since we last met.

The key test is whether the thing after since is a moment you could mark on a calendar or a clock. 2019 is a point. Monday is a point. "Six years" is not a point — it is a length — which is why "since six years" is wrong in every context.

I have lived here since 2019. — correct, a point
I have lived here since six years. — wrong, that is a length
She has been ill since Tuesday. — correct
We haven't spoken since the wedding. — correct, an event as a point
I have known him since we were at school. — correct, a clause

Since almost always pulls a perfect tense with it: present perfect, present perfect continuous, or past perfect. "I live here since 2019" and "I am living here since 2019" are both wrong for the same reason — neither is a perfect tense.

That coupling is so reliable that you can use it as a check. If you have written since and your verb has no have, has or had in it, look again.

Tip: Since can also be followed by a whole clause with a past-tense verb: "since I moved here", "since she left". The main verb still needs the perfect: "I have been happier since I moved here."

3For: the length of time

For answers "how long?" and it is the flexible one, which makes it both easier and more dangerous.

for + a DURATION. for two hours, for six years, for a long time, for ages, for a moment, for the whole afternoon.

Unlike since, for works with almost every tense. That is convenient and it hides a trap: the tense you choose changes whether the period is finished, and learners often pick one without noticing they have made that decision.

I worked there for five years. — past simple, I don't work there now
I have worked there for five years. — present perfect, I still do
I had worked there for five years before I left. — past perfect
I will be there for two hours. — future
I am staying for three days. — present continuous, arranged

The first two are the pair that matters. Same words, same duration, opposite employment status. Getting this wrong in a job interview does not produce a grammar error — it produces a false statement about your career.

For is sometimes dropped in speech with certain durations: "I've been here two hours" is natural and correct. This is optional omission, not an error, but do not drop it in writing.

Tip: For all my life, for the first time and for now are fixed phrases worth having ready; none of them behaves unexpectedly.

4Ago: counting back from now

Ago is the most mechanical of the three and the one with the strictest tense rule.

duration + ago, and it takes the PAST SIMPLE. two hours ago, three days ago, a long time ago, ten years ago.

Notice the word order. Ago comes after the time expression, not before it: "two hours ago", never "ago two hours". This trips up speakers of languages that put the equivalent word first, and it is worth drilling because the wrong order is instantly noticeable.

I saw him two days ago. — correct
I have seen him two days ago. — wrong, ago forbids the perfect
I ago two days saw him. — wrong word order
She left a moment ago. — correct
How long ago did you arrive? — correct question form

Ago and the present perfect can never appear together. Ago points at a finished moment in the past, and the present perfect requires an open period — they contradict each other. "I have finished it two hours ago" is wrong in every variety of English.

For counting back from a point in the PAST rather than from now, English uses before or earlier instead: "He had left two hours before." Ago is anchored to the present moment; before is anchored to a past one.

Tip: Ago is the easiest of the three to get right, because the rule has no exceptions: duration first, ago second, past simple verb.

5The tense each one forces

Here is the whole system compressed. If you memorise one table from this article, make it this one.

since + point → present perfect (or past perfect) — I have lived here since 2019
for + duration → any tense; the tense decides if it is over — I lived / I have lived
ago + duration → past simple, never the perfect — I arrived two hours ago
in + duration → future — I'll be there in ten minutes
during + noun → any tense — I slept during the film
by + point → deadline, often perfect — I'll have finished by six

The two rigid rows are since and ago, and they point in opposite directions: since forces the perfect, ago forbids it. That opposition is worth noticing, because it means since and ago can never appear in the same clause.

For is the only genuinely free one, and that freedom is why it produces subtler errors. Nobody writes an ungrammatical sentence with for; they write a grammatical sentence that says the wrong thing.

In is a common confusion with ago because both attach to durations. But in points forward — "in two hours" means two hours from now — while ago points back. "I'll finish in two hours" and "I finished two hours ago" are opposite directions.

Tip: Test yourself on the since/ago opposition specifically: since demands have, ago forbids it. Once that pair is automatic, most of this topic is done.

6Since as a conjunction, not a preposition

Since has a second job that has nothing to do with time, and learners who have drilled the time rule sometimes get confused when they meet it.

Since also means because: "Since you're here, could you help me?" "Since it was raining, we stayed in." This is a reason, not a starting point, and it carries no tense requirement at all.

Since 2019 I have lived here. — time
Since you asked, I'll tell you. — reason
Since he was late, we started without him. — reason
Since he left, nothing has been the same. — time

Distinguishing them is easy in practice: if you can replace since with because and the sentence still makes sense, it is the reason meaning. If you can replace it with from, it is the time meaning. The third and fourth examples above look identical in structure and split cleanly on that test.

The reason sense of since is slightly more formal than because and is common in writing. It also tends to introduce information the listener already knows, whereas because introduces new information. That nuance is subtle and rarely matters, but it is why "Since you're busy, I'll do it" sounds natural and "Because you're busy, I'll do it" sounds slightly heavier.

Tip: This is one of the few places in English where an ambiguity almost never causes a real problem. Context resolves it instantly, and native speakers do not notice it exists.

7For with the past simple: the case people miss

Many learners are taught "for goes with the present perfect" and stop there. That is a half-truth, and the missing half causes real misunderstandings.

For + past simple describes a completed period that is over. For + present perfect describes a period that is still running. Both are correct; they mean different things.

I lived in Berlin for six years. — I don't live there now
I have lived in Berlin for six years. — I still do
She studied Spanish for two years. — she stopped
She has studied Spanish for two years. — she is still studying
We waited for an hour. — the waiting ended
We have been waiting for an hour. — we are still waiting

Look at the last pair. "We waited for an hour" is what you say afterwards, in a story. "We have been waiting for an hour" is what you say while still standing there, usually with some impatience. The situation, not the grammar, tells you which you need.

The error here is not ungrammatical, which is why nobody corrects it. If you say "I have worked at Google for five years" when you left three years ago, your listener will believe you work there now, and nothing in the sentence will alert them.

This is the strongest argument for learning tense through situations rather than tables. The table says both are correct. Only the situation says which one is true.

Tip: Before saying a for sentence about yourself, ask whether the period is still running. That one-second check prevents the whole category.

8Before, from and until

Three more time words that sit close to these and get mixed in.

from — a starting point, but not with the perfect

From marks a start like since, but it does not demand a perfect tense and it usually pairs with to or until: "I worked there from 2015 to 2019." Use since when the period continues to now; use from when you are naming both ends of a finished period.

until / till — up to an end point

"I waited until six." "He won't arrive until Friday." Until marks where something stops. A common error is using it for duration: "I waited until two hours" is wrong; you want "I waited for two hours."

before — earlier than a point

"I'll call before six." It also serves as the past-anchored version of ago: "He had arrived two hours before." Ago counts back from now; before counts back from a moment in the story.

I worked there from 2015 to 2019. — finished period, both ends named
I have worked there since 2015. — still there
I waited until six. — end point
I waited for two hours. — duration
She left two hours ago. — counting back from now
She had left two hours before. — counting back from a past point

Tip: From … to and since are the pair worth separating carefully: from … to closes the period, since leaves it open.

9How long have you… versus how long did you…

The question forms are where all of this becomes visible, and where a wrong choice changes what you are actually asking.

"How long have you lived here?" asks about a period that is still running — you live here now. "How long did you live here?" asks about a finished period — you have moved away. Asking the wrong one is not a grammar mistake; it is asking a different question, and the answer you get will confuse you.

How long have you been learning English? — you still are
How long did you learn English? — you stopped
How long have you known her? — you still know her
How long ago did you meet her? — a single past moment
When did you arrive? — never "when have you arrived"

When cannot take the present perfect. "When have you arrived?" is wrong because when asks for a specific past point, and the present perfect refuses specific past points. Use "When did you arrive?"

The answers follow the same split. "I've lived here for six years" or "since 2019" answer the perfect question. "I lived here for six years" answers the past one. Matching your answer to the question's tense is a small thing that makes conversation noticeably smoother.

Tip: How long ago…? is always past simple, because it contains ago. That one is reliable enough to use as an anchor when the others feel uncertain.

10Why your first language pushes you wrong

Whether you make these errors, and which ones, is fairly predictable from the language you already speak.

Persian, Arabic, Turkish, German: the present tense leaks in

These languages describe an ongoing situation with a present tense: Persian «من پنج سال است اینجا زندگی می‌کنم», German "Ich wohne seit fünf Jahren hier". Both translate directly to "I live here since five years" — which is wrong twice over, in the tense and in since versus for. German speakers in particular are pushed wrong by seit, which covers both since and for.

French, Spanish, Italian: depuis and desde cover both

French depuis and Spanish desde translate as both since and for, so there is no signal telling you which English word to pick. "J'habite ici depuis six ans" becomes "I live here since six years". The fix is to check whether what follows is a point or a length before choosing the English word.

Russian, Polish: no perfect tense to map onto

Slavic languages carry completion in verb aspect rather than in a separate perfect tense, so there is no structure to transfer. Speakers tend to use the past simple everywhere, producing "I live here five years" or "I lived here since 2019".

If your language uses one word for since and for, assume you are making this error until you have specifically drilled it. It never blocks understanding, so it is never corrected, so it lasts for years.

Tip: Find your row above, then do the drill in section eleven twice. Knowing which direction you drift in is worth more than knowing the rule.

11Thirty sentences, corrected

Cover the right-hand side and fix each one before you look. These are drawn from the errors that appear most often in real learner English.

I live here since 2019 → I have lived here since 2019
I have lived here since six years → I have lived here for six years
I have finished it two hours ago → I finished it two hours ago
I am here since two hours → I have been here for two hours
I know her since we were children → I have known her since we were children
How long do you work here? → How long have you worked here?
When have you arrived? → When did you arrive?
I ago two days saw him → I saw him two days ago
She is ill since Tuesday → She has been ill since Tuesday
I waited until two hours → I waited for two hours
We are married for twenty years → We have been married for twenty years
I didn't see her since the wedding → I haven't seen her since the wedding
It's raining since this morning → It has been raining since this morning
I have worked there for five years, but I left in 2020 → I worked there for five years
How long ago have you moved here? → How long ago did you move here?
I study English since three years → I have been studying English for three years
He has left the office ten minutes ago → He left the office ten minutes ago
I am waiting for you since one hour → I have been waiting for you for an hour
She works here from 2018 → She has worked here since 2018
I worked there from 2015 to 2019 → correct as written
We haven't spoken for ages → correct as written
I will finish it ago two hours → I will finish it in two hours
Since you are here, can you help? → correct as written
He had left two hours ago (in a past narrative) → He had left two hours before
I have been here for two hours → correct as written
This is the first time I am here since 2019 → This is the first time I've been here since 2019
I haven't eaten since yesterday → correct as written
How long are you in Canada? (meaning so far) → How long have you been in Canada?
I lived in Berlin since six years → I lived in Berlin for six years
It has been a long time since we met → correct as written

Tip: Six of these are already correct. If you changed them, you are applying the rules too aggressively — which is a different problem from not knowing them.

12Collecting these from real speech

These three words appear in almost every conversation where someone talks about their own life, which makes them unusually easy to gather.

Any interview reaching "how long have you been doing this?" will give you a run of since, for and the present perfect within seconds. Any conversation with someone who has moved country will give you the same. You do not need to search for this input; you need to notice it when it arrives.

Listening also fixes something a rule cannot reach: these words are almost always unstressed and compressed in speech. For reduces to something like "fuh", and I've, we've and he's disappear into the following word. If you have only met these on paper, you will not hear the perfect tense at all — you will hear the past simple and conclude, wrongly, that native speakers ignore the rule.

Find an interview where someone describes their work or their life
Every time you hear since, note whether a point or a clause followed
Every time you hear for, decide from the tense whether the period is still running
Save the whole sentence, because the time word and the tense are one unit
Answer three questions about yourself out loud using since, for and ago once each

Tip: FlexiLingo puts interactive subtitles on YouTube and podcasts, so you can click any word to see its meaning and CEFR level and save it with the full sentence — which matters here because the time word alone teaches you nothing about the verb it demands.

Frequently asked questions

What is the difference between since and for?

Since marks the starting point — since 2019, since Monday, since I moved here. For marks the duration — for six years, for two hours. So "since six years" is always wrong, because six years is a length rather than a point. Since almost always demands a perfect tense; for works with any tense, and the tense is what tells your listener whether the period is over.

Why is "I live here since 2019" wrong?

The word since is right — 2019 genuinely is the starting point. The verb is wrong. Since demands a perfect tense, so it has to be "I have lived here since 2019". This is the single most common error with these three words, and it usually comes from a first language that describes ongoing situations with a present tense.

Can I use "ago" with the present perfect?

No, never. Ago points at a finished moment in the past and the present perfect requires an open period, so they contradict each other. "I have finished it two hours ago" is wrong everywhere; say "I finished it two hours ago". If you need to count backwards from a point in the past instead of from now, use before: "He had left two hours before."

Is "for" always used with the present perfect?

No, and believing that causes real misunderstandings. For works with any tense, and the tense carries the meaning: "I worked there for five years" means you have left, while "I have worked there for five years" means you are still there. Both are correct English; only one of them is true about you.

What is the difference between "How long have you lived here?" and "How long did you live here?"

The first assumes you still live here; the second assumes you have moved away. They are different questions, not different levels of politeness. Asking the wrong one will get you an answer that does not match what you wanted to know.

Does "since" always mean time?

No — it also means because: "Since you're here, could you help?" That sense carries no tense requirement at all. Telling them apart is easy: if you can replace since with because, it is the reason meaning; if you can replace it with from, it is the time meaning.