Imperative mood
The imperative mood is the verb form used to give a command, an instruction or a request, with the subject 'you' understood rather than stated — 'shut the door', 'hold the line'.
Shut the door. No subject appears, because the subject is whoever is being addressed. That absence is what makes the imperative the shortest complete sentence English offers, and the most direct.
It can be softened without leaving the mood: Please shut the door. Do shut the door. Shut the door, would you. It can also be negative: Do not shut the door. And it can include the speaker: Let us not pretend.
Three uses worth knowing.
Instructions. Recipes, manuals and how-to writing run almost entirely on imperatives, because they remove the reader from the sentence and leave only the action.
Second-person narration. A story written in the imperative — Take the second turning. Do not look at the house. — creates an unusual intimacy and an unusual unease, since the reader is being told what they are doing.
Rhetoric. The imperative is the natural close of a persuasive passage, and it stacks well with anaphora: Read it again. Read it slowly. Read it to the end. It is also the form that an apostrophe usually takes, since addressing the absent almost always means asking them for something.
In dialogue, a run of bare imperatives reads as authority, panic, or contempt, depending entirely on what the scene has already established.