Question

FRENCH II: Why do you change the re in faire to a t instead of a u?

Answer 1

When you change it to fait.

Answer 2

I assume that you are talking about the passe compose?

When you conjugate most /-re/ verbs, you would drop the infinitive ending and add /-u/(par exemple: je bois -> j'ai bu). Because faire is an irregular verb, when conjugating it into the passe compose it only takes the form of "fait". The only thing that needs to be changed is the helping verb of avoir conjugated to match the subject.

J'ai fait
Tu as fait
Il/elle/on a fait
Nous avons fait
Vous avez fait
Ils/elles ont fait

I hope that helped!

Answer 3

Irregular verb, as zangetsu said.
It comes from the Latin past participle factus, that became faict, and was later spelt fait.