The verb form IN a since-clause

Question

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?

Answer 1

It's a subtle aspectual difference, but in the following construction, the tea-drinker's drinking began no earlier than the moment he was seated, not no earlier than the moment he sat.

Since you've been sitting here, you've had ten cups of tea.

The meaning is not identical to:

You've had ten cups of tea since you sat down.

Compare:

You've had six beers since we sat down.

You've had six beers since we've been sitting here.

I would say that the second, with the progressive, in its temporal fuzziness, is a little less stark than the simple past. In a recrimination, say, the progressive would be a little less recriminatory. The simple past adds the nuance from the moment we sat down. The speaker is conscious of the time as a finite span with a definite beginning rather than as a continuum with an indefinite beginning.

Answer 2

Since you've been sitting here is an example of present perfect progressive tense. Since expresses a "starting point," as you say, and it does not mean "because" here. Yes, you could say the following if you wish:

You've drunk about ten cups of tea since you sat down.

In a lot of ways, this is clear enough, and most English speakers prefer to communicate in this simple way, especially in spoken language. However, perfect progressive allows you to be more expressive and informative, especially in written English. Consider your original sentence:

You've drunk about ten cups of tea since you have been sitting here.

This sentence makes it crystal-clear that your action of sitting down has been continuous and ongoing. It is clear that the tea was drunk while you were sitting down.

It's not the best example in the world. Perfect progressive might make more sense in certain sentences. For example:

I have studied hard for the exams.

This sentence suggests that, yes, I studied, but it's unclear whether I studied one night or for many months.

I have been studying hard for the exams.

This sentence is clearer, because it is implied that I have studied on separate occasions over a period of time.

So it is not always more natural to use just the perfect tense, because not all actions are completed easily. Sometimes it is more accurate and informative to use perfect progressive, like in this example:

I have chewed off all my fingernails since we have been studying for the exams.

Because it takes a long time to chew off all your fingernails, it makes sense to express that studying for the exams is an ongoing action that has been happening over a period of time.