A SENTENCE is defined by four things:
1) It has at least one subject.
2) It has at least one predicate.
3) It expresses a complete thought.
4) It has end punctuation.
A sentence might have other parts, as well, but if a group of words does not AT LEAST meet all four of these requirements, it is not a sentence.
There are four types of sentences (sentences that are built in a certain way) :
There are also for kinds of sentences (sentences that do different functions):