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.'
