A subordinating conjunction joins a subordinate clause and a main (principal) clause. The subordinate clause can be a noun clause or an adverb clause. Note that a subordinating conjunction is not used to connect an adjective clause to another clause.

Common subordinating conjunctions are: after, because, when, where, while, before, though, although, if, that, till, until, unless, than, as, since, as if, so, so that etc.

There are several different kinds of subordinating conjunctions.

Subordinating conjunctions expressing time

The train had left before we reached the station. (Principal clause – The train had left, subordinate clause – before we reached the station.)
Wait here till I come back. (Principal clause – Wait here, subordinate clause – till I come back.)
We left for our homes after the program had ended. (Principal clause – We left for our homes, subordinate clause – after the program had ended)
I have not seen since he left this city. (Principal clause – I have not seen him, subordinate clause – since he left this city)

Subordinating conjunctions of place

I shall go wherever I want to.

Subordinating conjunctions of purpose

We eat that we may live.
Play games in order that your health may improve.
Study hard lest you should fail.

Subordinating conjunctions of reason

Since he was not there I left a message with his mother. (Principal clause – I left a message with his mother, subordinate clause – since was not ready)
He was promoted because he was efficient.
As he was not ready we left without him.
Subordinating conjunctions of result or consequence

He was so weak that he could hardly walk.
She ate so much that she fell ill.
It was so hot that we couldn’t go out.

Subordinating conjunction of condition

I will go abroad if I get a good job.
He won’t help you unless you ask him.
It looks as if it might rain.

Subordinating conjunctions of concession or contrast

I will not forgive him even if he apologizes.
Although he is ill he is always cheerful.
Although he works hard he doesn’t earn much.

Subordinating conjunctions of comparison

He is taller than I am.
I am as healthy as you are.