The different between a phrase and a clause is that a clause has its own subject+predicate pair, i.e., a verb with or without objects and/or complements and a subject carrying out that verb. "Time flies" is a clause because the noun "time" is the subject that "flies." The same is true for "when time comes to a stop," but in this case the relative adverb <em>when</em> causes this clause to be dependent. There is no subject in "forgive and forget" and no verb in "before midnight."
