What best defines a dependent clause?

A dependent clause is characterized by its inability to stand alone as a complete thought. It includes both a subject and a verb, but it does not convey a complete idea on its own; instead, it often relies on an independent clause to provide context and meaning. This structure typically begins with a subordinating conjunction (like "because," "although," or "if") that signals its dependence.

In contrast, the other options describe different grammatical structures. One indicates a complete thought, which belongs to an independent clause. Another suggests a clause that expresses a complete idea with a subject and verb, which also refers to independent clauses. Lastly, a standalone sentence is synonymous with an independent clause and does not reflect the nature of a dependent clause. Therefore, the correct identification of a dependent clause as one that cannot stand alone emphasizes its role in complex sentence structures.