The typical answer to this is “I am,” but some argue that it’s not a complete sentence. However, if someone asked a man named Rupert if he was Rupert, he could reply, “I am” and it would make a complete sentence in my book.

But that’s not the whole answer. There is an even shorter sentence using an imperative with an implied subject (how’s that for an English terminology-filled sentence?) With “Go,” the “you” is implied. For example, if your wife wanted you to go with her to pick up some donuts and you were busy, she might say, “I really want to get some donuts, I’m starving!” and you might reply, “Go!” The implication being you never wanted to get donuts in the first place because you like ice cream more anyway and if you’re busy you’re probably doing something worthwhile and important and can’t be disturbed for such trivial matters as acquiring sweet pastries with holes in them, no matter how much of a waste of time your wife says your pursuits are. In short, “Go” is the shortest sentence in the English language that also has the longest implied meaning. Do you want to get get some ice cream? Go!

Ava pointed out in the comments that No is another viable alternative.