A simple sentence has one independent clause and no dependent clauses. An example is: The dog likes bacon.
A compound sentence has two or more independent clauses, or two simple sentences. An example is: He went to the game and he finished his homework. These are two independent clauses combined with a coordinating conjunction: "and." A compound sentence can also connect two independent clauses with a semicolon. I got a raise; I could finally pay off the gamblers.
A complex sentence contains at least one independent clause and at least one dependent (also called subordinate) clause. The clauses are usually joined by a subordinate phrase or conjunction. She decided to run the race unless the team physician said she could not. The clause "unless the team physician said she could not" is subordinate because it depends upon the other independent clause ("She decided to run the race"). The independent clause can stand alone but the dependent clause needs the independent clause in order to finish the thought.
Compound-complex (also called complex-compound) sentences contain two or more independent clauses and one or more dependent clauses. Although I don't speak Japanese, I plan to go to Japan and I will converse with anyone I meet.
The first milestone in lighting may have occurred when early man lit a torch from his cooking fire. This sentence seems like it is made up of two dependent clauses. Both "The first milestone in lighting may have occurred" and "when early man lit a torch from his cooking fire" seem like unfinished thoughts. However, "The first milestone in lighting may have occurred" can be considered an independent clause. Of course it occurred sometime, but it is a complete simple sentence to just say that it might have occurred. The "when" is the subordinating conjunction in the dependent clause "when early man lit a torch from his cooking fire." Since this sentence contains an independent clause and one dependent clause, it is a complex sentence. We could make it compound-complex by adding another independent clause. (The added independent clause is in bold.)
When early man lit a torch from his cooking fire, the first milestone in lighting may have occurred, and this would be a significant moment in history.
Answer : Complex Sentence-A complex sentence is a sentence with one independent clause and at least one dependent clause.