1. French –ir verbs
How to conjugate verbs
ending in –ir in French

2. Many French verbs ending in –ir
follow a regular pattern
The verb finir (to finish) is conjugated below.
You will have to memorize which verbs ending in –ir in
the infinitive follow the “normal” pattern of conjugation,
meaning you can just take the –ir ending off the infinitive
and add the following endings to the stem:
finir
je finis nous fin issons
tu finis vous fin issez
il,elle,on finit ils,elles fin issent
http://www.laits.utexas.edu/tex/gr/vir1.html

3. Some verbs ending in –ir are
irregular
This means you will have to memorize each form
separately, but they can be learned in groups like sortir,
partir and dormir:
sortir (to go out)
je sors nous sort ons
tu sors vous sort ez
il,elle,on sort ils,elles sort ent
Notice that the conjugations for the plural forms are all
regular, in other words they follow the normal process of
dropping the last two letters of the infinitive & adding the
appropriate endings.
http://www.laits.utexas.edu/tex/gr/vir3.html

4. Then we have verbs like “ouvrir”
Ouvrir means “to open” and it’s conjugated like a regular
–er verb:
ouvrir
j’ouvre nous ouvr ons
tu ouvres vous ouvrez
il,elle, on ouvre ils,elles ouvrent
Other verbs like ouvrir that you should know are couvrir (to
cover) and offrir (to offer)
http://www.laits.utexas.edu/tex/gr/vir2.html

5. Practice makes Perfect!
The only way to learn verb conjugations is to practice
often, much like you would to learn a piece of music.
The link below brings to an excellent website where you
can practice all French verbs, and gives you helpful
comparisons to English verbs:
http://www.conjuguemos.com