1. Five Parts to a Complete Sentence
Capital letter
Subject
Predicate
Complete thought
Terminal Punctuation
Spelling is also important.

2. Capital Letter
The first word of a complete sentence must start with a capital letter.

3. Subject
The subject of a complete sentence is who or what the sentence is about. The subject performs the action, if there is any, in a sentence.
Sentence fragments and incomplete sentences can lack subjects, as in the following examples:
Went to the store.
Drove very quickly.
Under the stairs.

4. Predicate
The predicate of a complete sentence is the action or what is going on in the sentence.

5. Complete Thought
A sentence is incomplete if it does not contain a complete thought.

6. Terminal Punctuation
A complete sentence is not complete without terminal punctuation.
Period
Question Mark
Exclamation Point