Learn Dutch: Sluit het restaurant meteen! - Close the restaurant immediately!

Because this is a command (the imperative mood). Just like in English, when you tell someone to do something, you don't need to include a subject pronoun like 'jij' or 'je'. You simply start with the verb.

How do we get the form sluit?

To make a command in Dutch, you use the stem of the verb. The full verb (infinitive) is sluiten (to close). If you remove the -en ending, you are left with the stem: sluit.

Why does meteen come at the end of the sentence?

In Dutch, time words like meteen (immediately) generally come after the specific object. Because het restaurant is a specific, known object (it has het in front of it), meteen is placed right after it.

How would I make this command sound more polite or formal?

To make a command formal, you can add the formal 'you' (u) directly after the verb. When using u, the verb needs a -t at the end. Since sluit already ends in a -t, the spelling stays the same, becoming: Sluit u het restaurant meteen!