Subordinating conjunctions show a relationship between an independent and a dependent clause; some of the relationships can be cause-and-effect, and contrast. Some examples of subordinating conjunctions are because, since, as, although, though, while, and whereas.
I can stay out until the clock strikes twelve.
Here, the subordinating conjunction is connecting the two ideas: I can stay out and the clock strikes twelve.
He can leave the house as long as he has cleaned his room.
The teacher said that he was amazingly creative.
The subordinating conjunction doesn’t need to go in the middle of the sentence. It must be part of the dependent clause, but it doesn’t matter whether the clause is the first or second on in the sentence.
Before he leaves, make sure his room is clean.
If the dependent clause comes first, you’ll need a comma; if the independent clause comes first, you probably won’t need a comma.
I drank a glass of water because I was thirsty.
Because I was thirsty, I drank a glass of water.
When she asked why I wanted a glass of water, I could only answer “Because”, because I was so thirsty.
I’ve used a comma in this sentence only to separate the repeated because; this makes it clearer to the reader.