An example sentence in a grammar book says:
You've drunk about ten cups of tea since you've been sitting here. (M. Swan, Practical English Usage, §522.3 since-clause: since we were at school)
Is this colloquialism, actually meaning 'since you (have(?)) sat down here'?
The use of progressive form in the since-clause is confusing to me. Doesn't it make the 'since' sound like saying 'because'? But it doesn't make sense as 'because' here. I believe the since is supposed to express a starting point of what the main verb says.
The book says
The tense in the since-clause can be perfect or past, depending on the meaning.
But actually I don't get the need for not only this perfect progressive but also the rest with the perfect:
I've known her since I've lived in this street.
We visit my parents every week since we've had the car.
Is it because 'live' has been chosen as the verb? Can't it be 'since I lived in this street'? Isn't it more natural to say 'since I moved in this street'?
It also shows "We visit my parents every week since we bought the car." Do you think it's because M. Swan just tried to make a comparison, he's chosen 'have' and said "since we've had the car", but actually it's more natural to say "since we bought the car"? Can't it be "since we had the car"... well, I suspect 'have' is not used like this, is it?