Subordinating Conjunction

Examples:

I know where you live

He failed because he did not work hard

I was not at home when he came

He may come if his father allows him

The first sentence may be split into two clauses, I know and where you live.

Evidently, these are not independent clauses. I know is the main or the Principal Clause and where you live is a Noun Clause object to the verb know. The Conjunction where is used to join a clause which is of a lower rank and is, therefore called a Subordinating Conjunction. For the same reason, the Conjunctions because, when and if are called Subordinating Conjunctions