Here's why:
* Phrase: A group of words that functions as a single unit but doesn't have a subject and verb. Example: "in the park"
* Clause: A group of words that contains a subject and a verb. Example: "When we saw him"
"When we saw him" is a dependent clause because it doesn't make complete sense on its own. It needs an independent clause to be a complete sentence. For example:
* When we saw him, he was smiling.
In this sentence, "When we saw him" is a dependent clause and "he was smiling" is an independent clause.