“Go.” looks too short to be a sentence, but it can be a complete one. In English, a complete sentence needs at least a subject and a verb. Imperative sentences (commands) are a special case because the subject is usually implied rather than stated.
When you tell someone “Go.” you are really saying “You go.” The verb is go, and the subject you is understood from context. That is why one word can still carry a full meaning and function as a complete sentence.
Here are a few more examples of complete one word imperatives:
- “Stop.” (You stop.)
- “Listen.” (You listen.)
- “Run.” (You run.)
It helps to contrast this with a one word statement like “Running.” On its own, that is usually not a complete sentence because it is not a finite verb with a clear subject. It can work in casual writing as a fragment, but it is not the same as an imperative.
So if you see “Go.” in dialogue or instructions, it is not missing anything. The grammar is complete, and the subject is simply understood.