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, …)