as, because, since and for

All four of these words can be used to refer to the reason for something. (For as, since and for referring to time, (see here), (see here), (see here).) There are some differences.

as and since

As and since are used when the reason is already known to the listener/reader, or when it is not the most important part of the sentence. As- and since-clauses often come at the beginning of sentences.

• As it’s raining again, we’ll have to stay at home.
• Since he had not paid his bill, his electricity was cut off.

As- and since-clauses are relatively formal; in an informal style, the same ideas are often expressed with so.

• It’s raining again, so we’ll have to stay at home.

because

Because puts more emphasis on the reason, and most often introduces new information which is not known to the listener/reader.

• Because I was ill for six months, I lost my job.

When the reason is the most important part of the sentence, the because-clause usually comes at the end. It can also stand alone. Since and as cannot be used like this.

• Why am I leaving? I’m leaving because I’m fed up! (not … I’m leaving as/since I’m fed up!)
• ‘Why are you laughing?’ ‘Because you look so funny.’

A because-clause can be used to say how one knows something.

• You didn’t tell me the truth, because I found the money in your room. (= … I know because I found …)

note

For more information about because, (see here).

for

For introduces new information, but suggests that the reason is given as an afterthought. A for-clause could almost be in brackets. For-clauses never come at the beginning of sentences, and cannot stand alone. For, used in this sense, is most common in a formal written style, and is becoming rare.

• I decided to stop and have lunch – for I was feeling hungry.

note

For for + object + infinitive clause (e.g. What I want is for everyone to have lunch), (see here).