Learn French: Ne nous salissons pas dans la forêt ! - Let's not get dirty in the forest!

This sentence is a negative command. In positive commands, the reflexive pronoun goes after the verb with a hyphen (like salissons-nous !). But in negative commands, the pronoun goes back in front of the verb, safely between ne and the verb: Ne nous salissons pas !

Wait, shouldn't there be two nous here?

In French commands (the imperative mood), you always drop the subject pronoun. A regular statement is Nous ne nous salissons pas (We are not getting dirty), but for a command like "Let's not get dirty", you drop the first nous (the subject) and only keep the reflexive pronoun.

What is the infinitive of salissons and why does it have a double "s"?

The verb is se salir (to get dirty). It is a regular -ir verb. For the nous form, regular -ir verbs take an -issons ending, which is where that double "s" comes from.

Can salir be used without the reflexive pronoun?

Yes! Without the reflexive pronoun, salir means "to dirty" something. For example, Ne salis pas tes chaussures ! means "Don't dirty your shoes!". By making it reflexive (se salir), the meaning changes to "to get (oneself) dirty".