What does je l ai mis sur mon mur mean in English?

"I put it on my wall."

Here's a breakdown:

* je: I

* l': it (masculine singular object pronoun)

* ai mis: put (past tense of mettre)

* sur: on

* mon: my

* mur: wall

It's important to remember that "l'" is a masculine pronoun, so it refers to a masculine noun.