Conjugation — five families

The stem is untouched in all six forms. That iss is part of the ending:

How to spot them: most are built from an adjective and mean "to become …":

grand → grandir to grow · rouge → rougir to blush · gros → grossir to put on weight · blanc → blanchir to whiten

English does exactly this with -en: red → redden, wide → widen, dark → darken, soft → soften. French -ir is English -en. Once you see that, this family stops being a list and becomes a pattern you already own.

Verbs in this family: finir choisir grandir réussir remplir obéir punir guérir réfléchir agir atterrir avertir établir nourrir ralentir rougir saisir vieillir salir maigrir grossir bâtir

And why ss rather than s? With one s it would be finisons — and an s between two vowels is pronounced like a z. Doubling it keeps the "s" sound. Same reason as poisson and dessert.

singular = short stem + s s t plural = long stem + ons ez ent

Family 2 ends in -mir · -tir · -vir. Family 3 ends in -re.

One trap: je suis is both "I am" (être) and "I follow" (suivre). Context tells you which.

They end in -ir but conjugate exactly like the first group, and their stem never changes:

j'offre · tu offres · il offre · nous offrons · vous offrez · ils offrent

Why they are different: their stem ends in two consonants in a row — offr · ouvr · cueill. offr + s would be offrs, which is unsayable, so the language slid an e in between.

Write the six forms in a column and draw a line around the ones whose stem changed — the shape is a boot:

nous and vous kept the older stem — the others drifted over the centuries and these two were left behind, because they carried the least stress. The boot is a fossil, not a rule.

English has the identical kind of fossil, and for the identical reason. I am · you are · he is are three different words for one verb; go takes went, a form borrowed from an entirely different verb. Frequent words resist regularisation, in every language. The boot verbs are French's am/are/is — and, as in English, they are the dozen you will use every day, so they get learnt whether you study them or not.

And their compounds are free: devenir · revenir · obtenir · comprendre · apprendre — each behaves exactly like the base verb.

And the largest boot group is right here: any verb ending in -enir. It has three stems, and they are the same three for all thirteen of them:

The endings are plain Set B — s s t · ons ez · ent. So learning venir hands you twelve more verbs for free:

venir · revenir · devenir · parvenir · prévenir · se souvenir · tenir · obtenir · appartenir · maintenir · retenir · soutenir · contenir

In speech this is the one thing you actually hear: the singular is nasal and ends on no consonant — il vient /il vjɛ̃/ — while the plural loses the nasal and you hear the n: ils viennent /il vjɛn/. That is the only audible singular/plural difference these verbs have.