Also, with the imperative the understood subject of the sentence is always you.

 This happens because the command or instruction is for the person the speaker is talking to, and that person is you.

See how in the following sentences, the speaker is talking to you, you are the recipient of the command, instruction or request:

Start the engine! Shift into reverse!

Imperative sentences are the only sentences in the active voice in English that do not require a subject (it is understood).