CONJUNCTIONS - Subordinating Conjunctions

A subordinating conjunction joins a clause to another on which it depends for its full meaning. Clauses that follow subordinating conjunctions are called subordinate clauses.

Examples of subordinating conjunctions are: after, because, if, that, though, although, till, before, unless, as, when, where, while etc.

"As he was not there I left a message with his mother."
"Answer the first question before you proceed further."
"I have not seen him since he was a boy."
"I must go now as I have some work to do."
"I think that he is trustworthy."

A subordinating conjunction together with its following clause acts like a part of the other clause.

"I will phone you when I arrive."
"I will phone you tomorrow."

The clause when I arrive is similar to tomorrow – it acts like an adverb in the clause I will phone you...

"He told me that he loved me."
"He told me a story."

The clause that he loved me is similar to a story – it acts like the object in the clause He told me …

Position of subordinate clauses:
Adverbial subordinating conjunctions and their clauses can go either at the beginning or end of sentences (depending on what is to be emphasized).

"You will pass if you work hard."
"If you work hard, you will pass."
"As he was not ready, we went without him."
"We went without him as he was not ready."
"When I am late, my father takes me to school."
"My father takes me to school when I am late."
"He works hard though he is weak."
"Though he is weak, he works hard."

Note that when a subordinate clause begins a sentence it is more often separated by a comma.

Leaving words out:
Words for repeated ideas cannot normally be left out in subordinating clauses. However, after if, when, while, until, unless and although, a pronoun subject and the verb be can often be dropped.

"We will wait if necessary." (= … if it is necessary.)
"When in Rome, do as Romans do." (= When you are in
Rome, …)

http://www.perfectyourenglish.com/grammar/subordinating-conjunctions.htm