Breakdown of Je la lui fais signer.
je
I
lui
him / to him / to her (indirect object)
faire
to do, to make
signer
to sign
Questions & Answers about Je la lui fais signer.
Why do la and lui go before fais instead of before signer?
In standard French, object pronouns usually go immediately before the verb they belong to. However, the causative faire construction acts as a tightly linked single verbal block. Because of this, any object pronouns for either faire or the infinitive (signer) must be placed before faire. You cannot say Je fais la lui signer.
Why is the person signing represented by the indirect pronoun lui instead of a direct pronoun like le?
This is a major rule of the causative faire structure: if the infinitive verb has a direct object (here, la representing the thing being signed), the person actually doing the action is forced into the indirect object slot. Since the document is the direct object, the person signing becomes lui (to him/her). If there were no document mentioned, the person would be the direct object: Je le fais signer (I am making him sign).
Why is the pronoun order la lui and not lui la?
When placing double pronouns before a verb, French follows a strict sequence. Third-person direct object pronouns (le, la, les) always come before third-person indirect object pronouns (lui, leur). Because both pronouns here are in the third person, it must be la lui.