Question

What is the subordinate clause of "The student who had not studied the lessons failed the test."?

Answer 1

A subordinate clause cannot stand alone as a sentence because it does not provide a complete thought. The reader is left wondering, "So what happened?" A word group that begins with a capital letter and ends with a period must contain at least one main clause. Otherwise, you will have written a fragment, a major error.
Break the sentence up around the 2 verbs.