Page 99 - Practical English Usage 3ed - Michael Swan, Oxford

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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 73, 208 and 522.) They are not used in the same way.
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.
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73
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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 ...) For more information about becallse, see 94.
for
For introduces new information, but suggests that the reason is given as an afterthought. Afor-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.
I decided to stop and have lunch - for I was feeling hungry.
as, when and while: simultaneous events
To talk about actions or situations that take place at the same time, we can use as, when or while. There are some differences.
'backgrounds': as, when or while
We can use all three words to introduce a longer 'background' action or situation, which is/was going on when something else happens/happened.
As I was walking down the street I saw Joe driving a Porsche.
The telephone always rings when you are having a bath.
While they were playing cards, somebody broke into the house. ~
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as, when and while: simultaneous events 73
Why am I leaving? I'm leaving because I'm fed up! (NOT ... "lett· /. ""tl II
:& n lflf;ltg as,SIIlt:e :& m J6 l:t;p!)
Why are you laughing?- Because you look so funny.
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