Example:
* Je veux faire (I want to do)
* Tu dois partir (You have to leave)
* Il faut aller (It's necessary to go)
Explanation:
In these cases, "faire," "partir," and "aller" are the infinitives of the verbs. The first verb ("veux," "dois," "faut") is conjugated to agree with the subject. The second verb remains in its infinitive form.
Important Note: There are some exceptions to this rule, particularly in complex sentences with subjunctive mood or past participles. But in general, you only conjugate the first verb in a sequence.