Le pluriel en -issent et la famille des verbes en -ir

Unité GR-24 · campagne 7 · finissent, choisissent : le pluriel qu'on entend.

Unit GR-23 showed that -er verbs hide their plural (il parle / ils parlent sound identical). The -ir verbs of the 2nd group do the opposite: they insert the syllable -iss- in every plural form, making the number loudly audible.

The family is big and regular: finir, choisir, réfléchir, grandir, jaunir, rougir, réussir, remplir, ralentir… — including the productive colour-and-change verbs (jaunir "turn yellow", vieillir "grow old"). One membership test: if the nous-form has -issons, the verb is 2nd group, full stop.

The trap is that not every -ir verb joins. A short list of very frequent verbs — partir, sortir, dormir, servir, sentir, mentir — looks identical in the infinitive but conjugates without -iss-: ils partent, ils sortent, ils dorment (never ils partissent). Their plural is still audible (il part [paʁ] / ils partent [paʁt] — the final consonant wakes up), but through the GR-23 mechanism, not through -iss-.

So when you meet an -ir infinitive, ask one question: finir-family or partir-family? The safe heuristic for B2: the partir-family is a closed club of about a dozen everyday verbs — memorise the six above — and essentially everything else in -ir takes -iss-. The practice mixes both families in the plural, where the choice is impossible to fake.

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