The counterpart of an independent clause. A dependent clause has a subject and verb, but it cannot stand on its own. It is not a complete sentence. A dependent clause begins with a DMW. For example, the following clause is independent:
Jim drove his car over the bridge.
but when a DMW is added to it, it becomes a dependent clause.
When Jim drove his car over the bridge…
The word “when” is a DMW and makes this clause dependent. It is called “dependent” because it must be attached to an independent clause in order for it to function as a complete thought (it is dependent on an independent clause).
Dependent clauses can function either:
When the DMW is a subordinating conjunction, the clause is usually adverbial; when the DMW is a relative pronoun, the clause is usually adjectival. See the presentation below or a video on the same subject.
See here for help diagramming clauses.