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 The difference in use between 'because', 'as', 'since' and 'for'
Agnes Leyen asks:

Could you please tell me the difference (in use) between because, as, since and for. I think it's very confusing.
Roger replies:
The present perfect is often used with since and for to denote periods of time up to the present. (Note that we do not use present perfect with expressions that refer to a time period that has finished, i.e. 'last week' or 'the day before yesterday'. Here the simple past is used: 'I went to the cinema three times last week.')

If you use since with the present perfect or present perfect continuous, you are signalling when something started. If you use for, you are signalling how long something has been going on. Compare:

  • 'She has been living in Holland since the summer of 1992.'
  • 'She has been living in Holland for the last nine years.'

That is one use of since and for.
But since and for can also be used in a similar way to as and because to give the reason for an action or a situation. However, there are important differences between them.

Because is used when the reason is the most important part of the sentence or utterance. The because clause usually comes at the end:
  • 'I went to Spain last summer because I wanted the guarantee of sunshine on every day of my holiday.'
As and since are used when the reason is already well known and is therefore usually less important. The as or since clause is usually placed at the beginning of the sentence:
  • 'As the performance had already started, we went up to the balcony and occupied some empty seats there.'
  • 'Since John had already eaten, I made do with a sandwich.'
For suggests that the reason is given as an afterthought. It is never placed at the beginning of the sentence and is more characteristic of written, rather than spoken English:
  • 'I decided to stop the work I was doing - for it was very late and I wanted to go to bed.'