A subordinate clause will not form a complete sentence by itself nor a complete thought all by itself. Therefore it is attached to a main clause and requires the main clause.

A subordinate clause may begin with a subordinate conjunction.

Ex: When (he turned on the flashlight),

THAT’S YOUR ANSWER!

Other examples:

Ex: After (I go to the movies),

Although (I went to the movies),

Because (I went to the movies),

If (I go to the movies),

Remember to use the comma after the subordinate clause when it precedes a main clause.

When the subordinate clause ends a sentence, usually no punctuation is used.

Ex: I will eat popcorn if I go to the movies.