A complete sentence has at least a subject and a verb and expresses a complete thought.
The basic pattern
The dog (subject) chased (verb) the ball (object).
- Subject: who or what does the action
- Verb: the action or state
- Object: who or what receives the action
Fragments
“Because it was late.” is a fragment: it has a subject and verb but does not finish the thought.
Simple, compound and complex
- Simple: I like tea.
- Compound: I like tea, and she likes coffee.
- Complex: Although I like tea, I drank coffee.