A conjunction that joins together clauses of unequal rank or importance is
called a subordinating conjunction. Read the following sentence:
Here that joins together two clauses of unequal rank. I told him is the main clause, and that he should consult a doctor is a subordinate clause which is the object of the verb told in the main clause.
A subordinating conjunction connects a noun clause or an adverb clause to some other clause. Note that subordinating conjunctions are not used to connect adjective clauses.
Subordinating conjunctions introducing noun clauses
The most common subordinating conjunction used for introducing a noun clause is that; others are if, when, whether, why and how.
Subordinating conjunctions introducing adverb clauses
Adverb clause of time
Examples are: before, since, till, after etc.
Adverb clause of reason
Examples are: as, because, since etc.
Adverb clause of purpose
Examples are: that, lest, in order that etc.
Adverb clause of result or consequence
Examples are: so…that
Adverb clause of condition
Examples are: if, unless, provided that
Adverb clause of concession
Examples are: though and although
Adverb clause of comparison
Example: than