Imperative sentences (sentences that give a command or an order) differ from conventional sentences in that their subject, which is always "you," is understood rather than expressed.

Stand on your head. ("You" is understood before "stand.")

Be careful with sentences that begin with "there" plus a form of the verb "to be." In such sentences, "there" is not the subject; it merely signals that the true subject will soon follow.

There were three stray kittens cowering under our porch steps this morning.

If you ask who? or what? before the verb ("were cowering"), the answer is "three stray kittens," the correct subject.