I'm not sure whether this is common knowledge or not, but since it did get mentioned in class the other week, I figured I'd ramble about it for a bit.
The imperative mood is used for commands, for example, "Give me money!" Sentences like this used to irritate me to no end, because I could never figure out how the sentence was grammatical. There's no subject! Just a verb and some objects. None of my pre-high school teachers could ever explain why this worked the way it did, and I had more or less given up on it by ninth grade, having filed it alongside Nessie and Big Foot in the unsolvables drawer in my head.
But the answer is really, really simple. There is a subject! That subject is "you." The sentence we're really saying is, "You, give me money!" or "You, wash the car!" or "You, make Mitch shut up!" But as we're all so very lazy that we drop the "you," so that when we say something in the imperative mood, it's just implied.