The imperative mood of a verb expresses commands and direct requests. It is always used in the present tense. Quite often in imperative sentences, the subject (you) is understood.
[You] Watch out for the broken step! [Use an exclamation point instead of a period after an urgent command.
[You] Take the A-train to 57th Street.
The other moods of verbs are indicative and subjunctive. (See also sentence types.)