A compound sentence is any sentence that contains more than one independent clause—for example:
We were going to take a walk, but then it started raining.
Both of the clauses in this sentence—(1) we were going to take a walk and (2) but then it started raining—are independent and could stand alone as sentences, so this is a compound sentence.
A sentence that contains one independent clause and one dependent clause is not compound—for example:
Because it started raining, we didn’t go for a walk.
The clause because it started raining is dependent on the main clause, so this is not a compound sentence.
The clauses in a compound sentences may be linked with a coordinating conjunction (as in the first example above), with a semicolon, or with an em-dash.
