Question

A subordinating conjunction usually introduces a(n) _____.

adjective clause
adverb clause
noun clause
prepositional phrase

Answer 1

Here's a chart listing the most common subordinating conjunctions ... FYI.
http://grammar.ccc.commnet.edu/grammar/conjunctions.htm#subordinating_conjunctions

Let me know what you think about how this type of clause is used.
http://grammar.ccc.commnet.edu/grammar/clauses.htm

Answer 2

Adverb Clause