How to conjugate irregular French verbs
On the previous page, we saw that irregular French verbs can be categorised according to four main verb features: their 'characteristic consonant' (if any), vowel change (if any), future stem (if irregular) and past participle form (often arbitrary). Now we'll see how to put this information to use in order to form irregular verbs in the different tenses.
The present tense is the most complex. The first stage is to work out the stem, which generally means removing the final syllable of the infinitive, but with some specific details as follows:
Note that in the case of -dre verbs, the preservation of the d in the stem is puraly a spelling complication, and the d is not pronounced in singular forms.
Another spelling complcation concerns n where this represents nasalisation on a vowel. For example, in peindre, the n isn't pronounced as a consonant1 but is really part of the representation of the vowel ein [æ̃]. As such, it is preserved in the written form of the stem peind-.
As a general rule:
If the characteristic consonant occurs in the infinitive, it is not present (at least in the spoken form) in the singular forms and so can be considered not to be part of the stem.
The interaction between the vowel change and the consonant is a little more complex in the case of -enir verbs, and it is probably easiest to learn these explicitly. (See the full verb table of venir and full verb table of tenir.)
In a small number of cases, the vowel change occurs in the singular forms (e.g. mourir > je meurs).
There are occasional spelling complications: -t is not written after -d (il perd) or -c (il vainc).
The plural forms are generally formed as follows:
Where the vowel change is from oi ([wa]), or in the case of u > eu, the vowel change distribution is slightly different: oi changes in the nous and vous forms; u > eu occurs just in the ils form (as well as in the singular).
As for virtually all verbs, the imperfect tense is formed as follows:
Remove -ons from the nous form and add the regular imperfect endings.
The future tense is formed by adding the future tense endings to the future stem: either to the infinitive minus any final -e or, in the case of a few specific verbs, to its irregular future stem.
The conditional, as for any verb, is based on the future tense forms but with the imperfect tense endings. See the page on the conditional for more information.
Irregular verbs don't generally have any special formation. As for virtually any verb:
This means that (both in written and spoken forms) the singular subjunctive forms are characterised by having the characteristic consonant and vowel change, whereas thes normally only occur in plural present tense forms.
Just a handful of irregular verbs have irregular subjunctive forms.