Learn French: Prends ce savon et lave-toi. - Take this soap and wash yourself.

This sentence uses an affirmative command with a reflexive verb. When you tell someone to do something with a reflexive verb like se laver (to wash oneself), the reflexive pronoun moves after the verb and attaches with a hyphen. Because this is the informal tu form, the pronoun te changes to the stressed form toi.

Why does prends end in an s but lave doesn't, if they are both commands for tu?

In the tu form for commands, -er verbs like laver drop their final s, making it lave. However, verbs that end in -re or -ir, like prendre, keep their final s. That is why you see prends with an s, but lave without one.

Why is it ce savon instead of le savon?

Ce is a demonstrative adjective meaning "this" or "that" for masculine singular nouns. While le savon means "the soap", using ce savon points to a specific item nearby, meaning "this soap".

How would the second half of the sentence change if I were speaking to a group of people?