I agree with the people who say that "I am" is a sentence.

My English teacher says that the word "am" denotes state of being, which can be seen as a predicate.

So therefore, there is a subject in the sentence ("I") and a predicate, in the form of a state of being ("am").

However, I believe that the word "Go" is a complete sentence because it is a command. My English teacher (she's awesome, I always say "my English teacher said this" or "my English teacher said that") thinks that commands are very rude because they are impersonal, not addressing a subject, although a subject is implied.