As, because or since?

Because is more common than as and since, both in writing and speaking. When we use because, we are focusing on the reason:

She spoke quietly because she didn’t want Catherine to hear.

We’ll come over on Sunday because David’s got to work on Saturday.

We often put the because-clause at the beginning of a sentence, especially when we want to give extra focus to the reason. We use a comma after the because-clause:

Because breathing is something we do automatically, we rarely think about it.

We can use a because-clause on its own without the main clause in speaking or informal writing:

A:

Would you like to go to school there?

B:

Yes.

A:

Why?

B:

Because my best friend goes there. (I would like to go to school there because my best friend goes there.)

We often use as and since when we want to focus more on the result than the reason. As and since are more formal than because. We usually put a comma before since after the main clause:

[result]I hope they’ve decided to come as [reason]I wanted to hear about their India trip.

[result]They’re rather expensive, since [reason]they’re quite hard to find.

We often use as and since clauses at the beginning of the sentence. We use a comma after the as- or since- clause:

Since everything can be done from home with computers and telephones, there’s no need to dress up for work any more.

As everyone already knows each other, there’s no need for introductions. We’ll get straight into the business of the meeting.

We use because, not as or since, in questions where the speaker proposes a reason:

Are you feeling unwell because you ate too much?

Not: Are you feeling unwell since you ate too much? or … as you ate too much?