Learn German: Wasch dir sofort die Hände! - Wash your hands immediately!

Because there is a direct object in the sentence: die Hände (the hands). When you do an action to a specific body part or object, that thing takes the accusative case (it is the thing being washed). The person receiving the action—you—becomes the indirect object, which requires the dative reflexive pronoun dir.

Why doesn't the sentence say deine Hände (your hands)?

In English, we say "wash your hands," but German handles this differently. The dative pronoun dir already tells us exactly whose hands are being washed. Adding deine would be repetitive, so German just uses the definite article die (the) when talking about body parts or clothing in reflexive sentences.

Why is the verb Wasch without an ending or an umlaut (two dots)?

This is an informal command (the imperative) for one person (du). To form it, you start with the present tense du wäschst, drop the -st ending, and—crucially for verbs like this—drop the umlaut so the ä goes back to a. This leaves you with the command form Wasch.

Why does sofort go in the middle of the sentence instead of at the end?