Which -ire verbs take -isc-, and why nobody can tell you a rule

Italian's third conjugation quietly splits in two, and textbooks tend to mention it once and move on. It deserves more than that, because it affects roughly two thirds of -ire verbs and there is genuinely no way to predict which side a verb falls on.

The split

Two perfectly regular -ire verbs, conjugated in the presente:

	dormire	finire
io	dormo	finisco
tu	dormi	finisci
lui/lei	dorme	finisce
noi	dormiamo	finiamo
voi	dormite	finite
loro	dormono	finiscono

Both are regular. Neither is an exception. They just follow different patterns, and nothing in dormire or finire tells you which.

Where the insert lands

Look at which forms take -isc-: io, tu, lui/lei, loro. And which don't: noi, voi.

That isn't arbitrary. It's the stress. In finisco, finisci, finisce and finiscono, the stress falls on the stem. In finiamo and finite it has moved onto the ending. The insert appears exactly where the stem is stressed. It exists to give the stress somewhere to land.

Once you see it, the pattern is memorable as a shape: the four corners take -isc-, the middle two don't. The same shape shows up in the congiuntivo presente and the imperativo, and nowhere else.

Where it doesn't appear

This is the half people miss. -isc- shows up in exactly three places:

• presente
• congiuntivo presente
• imperativo

In every other tense, finire and dormire conjugate identically. Finivo and dormivo. Finirò and dormirò. Finii and dormii. If you've been bracing for the insert to reappear in the imperfetto, you can stop.

So which verbs take it?

There's no rule. What there is, is a list, and it's shorter than it looks, because the high-frequency -ire verbs cluster heavily on the non--isc- side.

The common verbs that don't take it: dormire, partire, sentire, aprire, offrire, seguire, servire, soffrire, coprire, vestire, bollire, fuggire.

The common verbs that do: capire, finire, preferire, pulire, spedire, unire, costruire, chiarire, guarire, punire, restituire, suggerire, colpire, agire, stupire.

The full set is on the -isc- verbs page, ranked by frequency so you can learn the ones you'll actually meet first.

A small consolation

A verb that takes -isc- never stops taking it, and a verb that doesn't never starts. There are no verbs that go both ways, and no dialectal wobble to worry about. It's one binary fact per verb, learned once.

Which is why every -ire verb page on this site flags it in the header. Compare capire with dormire.