A simple sentence has one independent clause.  A compound sentence has two or more independent clauses joined by either a coordinating conjunction or a semicolon.  A complex sentence has one independent clause and one or more dependent clauses.  A compound-complex sentence has two or more independent clauses and one or more dependent clauses.

Simple sentence example—Birds fly.

Compound sentence example—Birds fly north in the winter, yet they return south in the autumn.

Complex sentence example—Whenever birds fly overhead, I cover my head.

Compound-complex sentence example—I need not look at my calendar to mark the season’s passing, for I see the birds fly to the pond in front of my house whenever the ice has melted in the spring.

Mrs. J <><

P.S.  The sentence “Birds fly” is my favorite due to my sixth grade English teacher, Mrs. Hamilton.  I think if it was good enough for me to learn, then it’s good enough for my students to learn too.  Thanks, Mrs. Hamilton!