Hello, everyone:
I've asked a question in http://www.englishforums.com/English/QuestionAboutTenseUsageSinceClause/gwxpb/post.htm , but I find out there are more than I need to ask, so I think it would be better to write a new post.
Recently I saw several sentences using simple past and present perefect in their since clauses. They are really confusing to me. I really hope you could help me comfirm their meaning. It may be very easy for you, but I've been wondering what their real meanings are for many days and couldn't find a clue. Please take a look at the following sentences:
1. It's a long time since he has lived there.
2. It's a long time since he lived there.
3. It is already 6 years since he have lived in New York.
4. It is already 6 years since he lived in New York.
I think those four sentences suggest "He doesn't live here/in New York now.", right? If my understanding is true, then please see the next pairs of sentences:
5. She has written to me frequently since I have been ill. (Does it suggest "I am still ill now"? If not, what does it suggest?)
6. She has written to me frequently since I was ill. (Does it suggest "I am not ill now."? If not, what does it suggest?)
If my understandings about sentence 5, 6 are right, then I think that's very tricky. Because sentence 1 and 3, which also use "have done" in their since clauses, suggest the same as sentence 2 and 4, which use simple past in their since clauses. However, although sentence 5 also uses "have done" in its since clause like sentence 1 and 3 do, sentence 5 does not suggest the same thing as sentence 6 does. Why is that? Is it because sentence 1, 2, 3, 4 use "It is + a period of time" structure? If so, I would be very amazed that simply a minor change like this would completely change the meaning of "have done" in the since clause. What do you think?
Here are some similar pairs of sentences:
7. Since she has been at Beijing University, it has gone through great changes. (I think it suggests "She is still at Beijing University now", am I right? If not, what does it suggest?)
8. Since she was at Beijing University, it has gone through great changes. (I think it suggests "She is not at Beijing University now", am I right? If not, what does it suggest?)
9. It's already 5 years since he has been in the army.
10. It's already 5 years since he was in the army.
I think sentence 9 and 10 both suggest "He is not in the army now", am I right? (Because "It is + a period of time" structure is used in the main clauses.) If not, what does it suggest?
11. They have been happy since they have lived in London. (I think it suggests "They still lives in London now", am I right? If not, what does it suggest?)
12. They have been happy since they lived in London. (I think it suggests "They don't live in London now", am I right? If not, what does it suggest?)
13. It's a long time since they have lived in London.
14. It's a long time since they lived in London.
I think sentence 13 and 14 both suggest "They don't live in London now", am I right? (Because "It is + a period of time" structure is used in the main clauses.) If not, what does it suggest?
I know those questions might be a piece of cake for native speakers, but they're totally confusing to me. I've searched all my grammar books for the answer but couldn't find one. I would appreciate it if you could help me solve my doubt. Thank you for reading this post!!