A clause is a group of words that contains a subject and a predicate.
Clauses are of two kinds: independent and dependent. Independent clauses are also called main clauses. Dependent clauses are also called subordinate clauses.
Main or independent clauses can stand alone as a complete sentence. A sentence that contains one clause is called a simple sentence:
The monkeys sang.
Dr. Doolittle accompanied them on the piano.
Independent clauses can be joined by conjunctions to make one sentence:
The monkeys sang, and Dr. Doolittle accompanied them on the piano. (main/independent clauses)
Dependent or subordinate clauses cannot stand alone as a sentence. They must be attached (subordinated) to a main clause:
I will take you to the library when I have time. (subordinate clause)
Because you have kept your promise, I will be your friend. (subordinate clause)
Dependent clauses begin with a word like because or when that explains its connection with the main clause.
Note: When analyzing a sentence that contains an independent clause and one or more dependent clauses, I usually refer to the independent clause as “the main clause” and to the dependent clauses as “subordinate clauses.”