A subordinate, or dependent, clause is similar to a main clause, but it is not a complete thought.
How to Spot a Subordinate Clause
Subordinate clauses typically begin with subordinating conjunctions, relative pronouns, or relative adverbs.
Subordinating Conjunction Examples
The seals sing mournful songs at sunset = main clause
The seals is the subject, and sing is the verb.
because they fear the night = subordinate clause
They is the subject, and fear is the verb. It is subordinate because it begins with the subordinating conjunction because.
Relative Pronoun Examples
The cow. . . is a master hypnotist = main clause
The cow is the subject, and is is the verb.
who let me win the staring contest = subordinate clause
Who is the relative pronoun and subject, and let is the verb.
Aliens prefer dabbing = main clause
Aliens is the subject, and prefer is the verb.
when there is a full moon = subordinate clause
There is the subject, and is is the verb. This clause starts with the relative adverb when, so it is subordinate.