How do you say there is a monster under my bed in french?

There are a few ways to say "There is a monster under my bed" in French, depending on the level of formality and the desired nuance:

Formal:

* Il y a un monstre sous mon lit. (Most literal translation)

* Il existe un monstre sous mon lit. (Emphasizes the existence of the monster)

Informal:

* Y'a un monstre sous mon lit! (More casual, uses slang)

* J'ai un monstre sous mon lit! (More personal, emphasizes your fear)

You can also use the more descriptive phrase "Il y a une bête horrible sous mon lit" (There is a horrible beast under my bed).

The choice of which phrase to use depends on the context of the situation and your intended audience.