A sentence with one main clause is called a simple sentence:
She goes to school.
A sentence with two or more main clauses is called a compound sentence:
She goes to school, but she doesn't like it.
A sentence with a main clause and one or more subordinate clauses is called a complex sentence. In the example below, the subordinate clause is in green; the main clause is in blue:
When she leaves school, she's going to be an actress.
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