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.”

Remember: Mastery of the parts of speech is a prerequisite for understanding clauses and other parts of the sentence.