There are three groups of verbs in French. In the second one, there are regular verbs ending in -ir.
Tu dois nourrir le chat.
You need to feed the cat.
Rule 1: To conjugate in the present, one must replace -ir with one of the following endings: -is, -is, -it, -issons, -issez, -issent.
Here's an example with the verb finir (to finish):
|Je finis||Nous finissons|
|Tu finis||Vous finissez|
|Il, elle, on finit||Ils, elles finissent|
Note: not all verbs ending in -ir are regular. You have to pay close attention.
Rule 2: Note that here, the three verb forms in the singular are pronounced the same way. The singular and plural forms of the third person are clearly distinguishable (finit vs. finissent).