Bonjour Francis,

In “Je me suis fait une promesse”, ‘fait’ doesn’t agree with its subject because, in this instance, it is part of French verbs which are occasionally pronominal. As such it follows the same rule of agreement that is used for Le Passé Composé with ‘avoir’: agreement if the verb is preceded by a direct object.

To find it out you need to do the following: is there a direct object (which can be the reflexive pronoun or something else)? And ask the usual question: verb + qui? or verb + quoi? Is it before or after the verb?

- agreement if direct object and reflexive pronoun match one another

Elles se sont insultées = they insulted each other
insulter qui? → se = reflexive pronoun → in French before the verb =                                                                                                         agreement
insulted who? → each other = se → In French before the verb = agreement

- no agreement if direct object and reflexive pronoun don’t match

Elle s’est fixé des objectifs = she set herself some objectives
fixer quoi? → des objectifs → in French after the verb = no agreement
set what? → some objectives → in French after the verb = no agreement

Je me suis fait une promesse = I made myself one promise
faire quoi? → une promesse → in French after the verb = no agreement
make what? → one promise → in French after the verb = no agreement

I hope this is helpful.

Bonne journée!