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Dependent Clauses

If a sentence has two Independent Clauses, each one can be free of the other.
 Harold liked to drive, (and) they went to the store. 

Either one can stand by itself. See?
Harold liked to drive.
They went to the store.

However, if we say:  Harold liked to drive WHEN they went to the store.  We use "when" instead of "and" and that changes things a lot.  Now there is a specific time that Harold like to drive. "When" is a conjunction that makes "they went to the store" a Dependent Clause.

Note: Sentences with two Independent Clauses -- with Conjunction like "and" or "but" -- are called "Compound Sentences." However, when one of the clauses is Dependent, we have a "Complex Sentence."

Compound Sentence: They drink tea and I drink coffee.
Complex Sentence:    When they drink tea, I drink coffee.

Often just one word makes the clause Dependent, and makes the sentence Complex. Look for words like "When" and "While" when you are using the Past Continuous.....:)

Now you've had enough...