A practical english grammar - Vyssaja skola 1978

Clauses of cause
Sound symbols

The idea of cause in an adverbial clause is expressed by because, since, as, and for. They occur in descending order of frequency, because being the most frequent and for the least frequent. Since usually means that the cause is evident; as expressing cause is not often heard in American English; for is literary and somewhat archaic.

I brought an umbrella because I thought it might rain.

Since it looks as though it might rain, I brought an umbrella. As it was raining, I brought an umbrella.

I brought an umbrella, for it was raining.

Clauses with because, since, and as may occur either before or after the main clause; those with for must follow the main clause.

Now that is similar in meaning to since; it points out facts so that conclusions can be drawn from them.

Now that school is over, you will have more free time.