Simple, Compound, and Complex Sentences

Subject: Who or what the sentence is about
Predicate: The action that the subject is doing

Simple Sentence: An independent clause (a complete thought) ex. The boy yawned.
Can be added onto with modifiers (descriptive words) ex. The tall boy in the back of my science class yawned loudly.
You may also have a compound subject. ex. The tall boy and his friend yawned.
You may also have a compound predicate. ex. The tall boy yawned and stretched his arms in science class.

Compound Sentence: A sentence combining two or more independent clauses (complete thoughts) that are joined by a coordinating conjunction and a comma or by a semicolon (;)
Coordinating conjunction and comma ex. My brother forgot his homework, and he had to call home.
Semicolon ex. My brother forgot his homework; he had to call home.

Complex Sentence: One independent clause and one or more dependent clauses (incomplete thought). If the the dependent clause comes first then it must be followed by comma.
Dependent clause first ex. Since we got to the concert late, we had to make our way to our seats in the dark.
Independent clause first ex. We had to make our way to our seats in the dark since we got to the concert late.

Remember a single word can change a clause from an independent clause to a dependent clause.
Dependent clause: Since we got to the concert late
Independent clause: We got to the concert late.
Adding the word since changes the word from an independent clause to a dependent clause.