Updated: May 28, 2020
A simple sentence contains a subject and a verb and expresses one complete thought.
I eat pizza in the morning.
I eat pizza in the morning is also an independent clause. It is a complete phrase which can stand alone.
INDEPENDENT CLAUSE (MAIN CLAUSE)
An independent clause is a phrase that can stand on its own as a complete idea or thought. It doesn't need or depend on another clause to be complete. It must contain a subject, a verb.
DEPENDENT CLAUSE (SUBORDINATE CLAUSE)
A dependent clause is a phrase with a subject and a verb, however, it does not express a complete thought so it is not a sentence and can't stand alone.
A compound sentence is two simple sentences with two subjects and two verbs joined together. A compound sentence contains two complete thoughts. It has two independent clauses. You can join two simple sentences into a compound sentence with a comma followed by a coordinating conjunction. Compound Sentences can make your writing richer and more colourful.
Simple sentence: I eat pizza in the morning.
Simple sentence: My brother eats hamburgers in the morning.
Compound sentence: I eat pizza, and my brother eats hamburgers in the morning.
A complex sentence combines an independent clause with one or more dependent clauses. A complex sentence always has a subordinating conjunction (after, although, because, since, when) or a relative pronoun (that, which, who).
complex sentence: I am overweight because I eat pizza in the morning.