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.
Think of words ending in -GRY. Angry and hungry are two of them. There are only three words in the English language. What is the third word? The word is something that everyone uses every day. If you have listened carefully, I have already told you what it is.
The riddle states, “There are only three words in the English language. What is the third word?” The third word of the phrase “the English language” is of course “language.” Don’t get angry at me, I didn’t make it up :)
The most popular answer for this teaser is ‘today’. Of course this only works if you’re reading it on that particular date or if the date is not specified. Another option is to respond literally with ‘a date’. Lastly, certain dates could be squeezed into letters using Roman numerals if you abbreviate the year to 2 numerals. For example, January 20th, 2011 would be I XX XI.