Final answer:
Subordinate clauses lack a subject and a verb, and are classified as prepositional, verbal, appositive, and absolute. They cannot stand alone as complete sentences and depend on the main clause for meaning.
Step-by-step explanation:
Subordinate clauses are subordinate because they lack a subject and a verb. They are classified into four types: prepositional, verbal, appositive, and absolute. Subordinate clauses begin with subordinating words or phrases such as although, because, when, since, or while. Unlike main clauses, subordinate clauses cannot stand alone as complete sentences and always rely on the main clause for context and meaning.