How do you say in french let me know?

There are a few ways to say "let me know" in French, depending on the context and level of formality:

Formal:

* Faites-moi savoir: This is the most formal way to say "let me know."

* Tenez-moi au courant: This literally means "keep me informed," and is another formal way to say "let me know."

Informal:

* Fait moi savoir: This is the informal equivalent of "Faites-moi savoir."

* Tiens-moi au courant: This is the informal equivalent of "Tenez-moi au courant."

* Dis-moi: This means "tell me" and is a very casual way to say "let me know."

More specific:

* Faites-moi savoir si... (Let me know if...)

* Tenez-moi au courant de... (Keep me informed of...)

* Dis-moi quand... (Tell me when...)

* Je vous tiens au courant. (I will keep you informed.)

Examples:

* "Faites-moi savoir si vous avez des questions." (Let me know if you have any questions.)

* "Dis-moi quand tu arrives." (Tell me when you arrive.)

* "Tenez-moi au courant de vos progrès." (Keep me informed of your progress.)

The best option for you will depend on the specific situation and your relationship with the person you are talking to.