Dependent Clauses cannot stand by themselves and make good sense. They must be combined with an independent clause so that they become part of a sentence that can stand by itself. (Review the section on Commas Usage for advice and plenty of exercises on the punctuation requirements when dependent and independent clauses are combined.) Unlike independent clauses, which simply are what they are, dependent clauses are said to perform various functions within a sentence. They act either in the capacity of some kind of noun or as some kind of modifier. There are three basic kinds of dependent clauses, categorized according to their function in the sentence. Remember that a dependent clause always contains a subject and a verb, but it cannot stand by itself.
Adverb clauses provide information about what is going on in the main (independent) clause: where, when, or why. "When the movie is over, we'll go downtown." or "John wanted to write a book because he had so much to say about the subject."
Adjective clauses work like multi-word adjectives. "My brother, who is an engineer, figured it out for me." or "The bridge that collapsed in the winter storm will cost millions to replace." A special kind of adjective clause begins with a relative adverb (where, when, and why) but nonetheless functions as adjectivally.
Noun clauses can do anything that nouns can do. "What he knows [subject] is no concern of mine." or "Do you know what he knows [object]?" or "What can you tell me about what he has done this year [object of the preposition "about"]?"
noun=subject What they did with the treasure remains a mystery.
Whatever you want for dessert is fine with me.
That you should feel this way about her came as a great surprise to us.
noun-obj Juan finally revealed what he had done with the money.
Her husband spent whatever she had saved over the years.
I don't know what I should do next.
noun-prep In fact, he wrote a book about what he had done over the...