As, because or since?

As, because and since are conjunctions. As, because and since all introduce subordinate clauses. They connect the result of something with its reason.

result	reason
He decided to go to the conference in Barcelona,	as he was in Spain anyway.
Are you angry with me	because I opened the letter?
Bilardo coaches his team by telephone, (Bilardo was the coach of the Argentinian football team)	since half of them play in Italy, France or Spain.

Because

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

Warning:

We don’t use a because-clause on its own in formal writing:

In 1998, the government introduced a new import tax because people were importing cars from abroad.

Not: … a new import tax. Because people were importing cars from abroad.

Cos

We often shorten because to cos/kəz/ or /kɒz/ in informal speaking and writing:

I’m laughing cos I’m so happy.

As and since

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 comeas[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?