Like all clauses, this word group contains both a subject and a verb (plus any accompanying phrases or modifiers), but unlike the independent clause, the subordinate clause cannot stand alone; it does not express a complete thought. Also called a dependent clause, the subordinate clause depends on a main clause (or independent clause) to complete its meaning. Easily recognized key words and phrases usually begin these clauses.
For example: although, because, unless, if, even though, since, as soon as, while, who, when, where, how and that.
Example: Yellowstone is a national park in the West that is known for its geysers. underlined phrase = subordinate clause