Je la fais réparer: pronouns and le or lui

Where the pronouns go

Because nothing may break the block, an object pronoun cannot sit in front of the infinitive it belongs to. It goes in front of faire.

Not je fais la réparer. In a compound tense the pronoun goes in front of the auxiliary, as with any verb.

The one exception is the usual one: after a positive command, the pronoun follows faire.

• Fais-le entrer ! - Show him in!
• Faites-moi rire ! - Make me laugh!

One object or two: le or lui

Count the objects. When the infinitive has no object of its own, the person is the direct object: le, la, les.

• Je le fais travailler. - I make him work.
• Je les fais attendre. - I keep them waiting.

When the infinitive has its own object, that object takes the direct slot, and the person becomes indirect: lui, leur. With a noun, the person takes à.

Where à would be unclear, the doer can come in with par instead: Je fais réparer la voiture par le voisin. - I'm having the car repaired by the neighbour.