I have not heard from him since he lived here. (Activity)
How long is it since you were in London? (State)
John is now with his parents in New York city, it is already three years since he was a headmaster. (State)
He has studied very hard since he came to our school. (Accomplishment)
I have learned to be dangerous upon points of honour since I served the Spaniard. (Activity)
It is three weeks since you have been here. = It is three weeks since you were here.
These are all taken from a paper written by a Chinese college teacher who teaches English.
He said different verbal aspects in these since clauses could lead to different interpretation:
#1 implies some time ago he left here and I haven't heard from him since then.
#2 again implies you haven't been living in London and I ask for how long you haven't been living in London.
#3 implies he resigned as a headmaster three years ago.
#4 is easy. He has been studying hard in our school.
#5 is different than #1, 2, 3. It means I have learned to ... since I began to serve the Spaniard and during the time I have served the Spaniard.
#6 is an idiosyncrasy. He said that in the "It's some time since clause" pattern the use of present perfect or the past simple makes no difference in meaning: You haven't been here for three weeks.
I'm not very confident about his explanations, especially #6. I think at least in #6 the choice of tense does have an influence over the meaning.
And why would #1, 2, 3 have the opposite meaning from #5?
I'd like to hear what a native speaker would say about it.