A subordinate clause, also known as a dependent clause, is a group of words that contains a subject and a verb but does not express a complete thought. It cannot stand alone as a sentence and relies on a main (or independent) clause to provide context and meaning. Subordinate clauses often begin with subordinating conjunctions (like "because," "although," "if," "when," "since," etc.) or relative pronouns (such as "who," "which," "that").
For example, in the sentence "Although it was raining, we went for a walk," the phrase "Although it was raining" is a subordinate clause, as it does not convey a complete idea on its own.