A simple sentence only has one main idea and clause.
Campbell ate pizza.
Connor hit the ball.
Compound sentences have two main ideas. They have two independent clauses that have equal importance. You will always find a coordinating conjunction (FANBOYS) or a semicolon separating a compound sentence.
Yangeni bought a new pair of shoes and Luke played a soccer game.
Jamison decided to swim without any clothes at school; swimming in the nude is not acceptable and he faced a disciplinary hearing.
A complex sentence is where there are subordinate clauses linked to the main idea. One clause in the sentence is the main idea.
Unless the students do the work, they are not leaving the classroom!
All the fans were doubting that Manchester United would beat Chelsea until Paul Pogba scored a hat-trick against them.