Learn German: Öffnet die Tür nicht!

This command is directed at a group of people you are on informal terms with, like two or more friends or children. It uses the ihr (plural informal you) form of the imperative.

Why is there no pronoun like ihr or Sie in this sentence?

In the ihr form of the imperative, the pronoun ihr is completely dropped. You just start the sentence with the conjugated verb: Öffnet (Open!). If this were formal, we would keep the pronoun and say Öffnen Sie.

Why does öffnet end with -et instead of just -t?

Normally, the ihr form ends in just -t (like macht or geht). But the root of the verb öffnen ends in an n (öffn-). To make it easier to say out loud, German adds an extra e before the ending, making it öffnet.

Why is nicht placed at the very end of the sentence?

In a German command, the verb comes first and its direct object (die Tür) usually comes right after it. The word nicht (not) gets pushed to the end to negate the entire action of opening the door.