Learn French: Ne te penche pas par la fenêtre, c'est dangereux ! - Don't lean out the window, it is dangerous!

In an affirmative command, the reflexive pronoun comes after the verb with a hyphen (penche-toi). But in a negative command like this one, the pronoun goes before the verb and uses the normal pronoun form (te), placed between ne and the verb: ne te penche pas.

Why is there no "s" at the end of penche, since it's an "-er" verb used with the "tu" form?

In the imperative (command) form, regular -er verbs lose their final -s for the tu form. So, while you would say tu te penches (you lean) in a normal sentence, the command drops the "s" to become just ne te penche pas.

Why does the sentence use par (by/through) to mean "out"?

In French, when you talk about looking, leaning, or throwing something "out" of a window, you use the preposition par, which literally means "by" or "through". Saying par la fenêtre is the natural way to translate "out the window".

Why do we use c'est dangereux instead of il est dangereux?

We use c'est followed by a masculine singular adjective to describe a general situation, action, or fact (like the act of leaning out the window). You would use il est if you were describing a specific masculine noun, like a bridge or an animal.