Simple sentences consist of just one independent clause; a simple sentence requires only one punctuation mark at the end (a full stop, exclamation, or question mark).
The essay was late.
Compound sentences are made by joining simple sentences. We join sentences that are closely related in content to make the writing more fluid. We can join simple sentences with a comma and a word such as: and, but, so, yet.
The essay was late, so he lost marks.
Complex sentences are made when we combine an independent clause with a dependent clause. The dependent clause in the following example is in italics.
Because his essay was late, he lost marks.