Subordinating conjunctions are transitions words that signal:

Subordinating conjunctions are transition words that signal the beginning of a subordinating clause. (A subordinate clause is a clause that depends on another clause and cannot stand alone.)

Examples of subordinating conjunctions are:

because, when, as, as long as, unless, until, after, before, although, provided that, etc.

There are 3 types of conjunctions:

subordinating conjunctions: connect an independent/main clause to a dependent/subordinating clausecoordinating conjunctions: connect two clauses of the same typecorrelative conjunctions: connect two clauses of the same type but always come in pairs, for example: both..and, neither...nor, etc.)

The subordinating conjuctions can introduce:

a noun clause (the subordinating clause acts as a noun) an adjective clause (the subordinating clause acts as an adjective, it describes a noun)an adverb clause (the subordinating clause works as an adverb)  

It provides additional information that narrows the focus of the test

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