Subordinating Conjunctions
Subordinating conjunctions, on the other hand, connect grammatical structures (usually clauses) of unequal rank. A subordinating conjunction introduces a clause that is dependent on the main clause. The dependent clause can be introduced preceding the main clause in an introductory role, or it may follow the main clause to add meaning to the sentence. Some common subordinating conjunctions include although, as, before, until, when, and while. You can find a more complete list on the English Plus Website.
As with coordinating conjunctions, the subordinate counterparts have a few punctuation rules to keep in mind. For example, when introducing a clause that precedes the main clause, insert a comma between the final word of the introductory clause and the first word of the main clause.
Before the girl returned home, she walked to the park with her friends.
However, if the subordinating conjunction follows the main clause, a comma is not necessary.
Shane remained at the library until his daughter called to ask for a ride home.
In this case, adding a comma was unnecessary because the subordinate clause was crucial to the meaning of the sentence.