Learn French: La robe que tu as fait coudre est parfaite. - The dress that you had sewn is perfect.

This is the core exception of your current lesson! Normally, in the passé composé with avoir, the past participle agrees with a direct object placed before the verb. Since la robe comes before the verb (represented by que), you would expect faite. However, when faire is followed by an infinitive (like coudre) to mean "to have something done," its past participle fait is strictly invariable. It never adds an -e or an -s.

How does fait coudre translate exactly to "had sewn"?

This uses the French causative construction: faire + an infinitive. It expresses that the subject caused an action to happen rather than doing it themselves. tu as fait coudre literally means "you made sew," which translates to English as "you had [it] sewn" or "you got [someone] to sew [it]."

Why do we use que instead of qui here?

que is used because the dress is the direct object of the relative clause (you had the dress sewn). qui is used for subjects (the person or thing performing the action). Since tu (you) is the subject performing the action of as fait, the relative pronoun replacing the dress must be the object pronoun que.

Why does get an at the end if doesn't?