There are three main candidates for the shortest sentence in the English language.
- Okay. - Sometimes used as a self-contained response, e.g., an implied agreement to complete an action or series of actions: How about you cook lunch and I'll hoover the bedroom?" he said. She nodded, 'Okay. Arguably not technically a sentence because it lacks a verb and a subject.
- I am. - Only meaningful as a sentence when used as an answer to a question: Who is going to the Moon this morning? I am.
- Go! - Has an active verb (t0 go), an implied subject (second person singular or plural) and a clear meaning.