1) since +过去一个时间点 (如具体的年、月、日期、钟点、1980, last month, half past six)。
I have been here since 1989.
2) since +一段时间+ ago
I have been here since five months ago.
3) since +从句
Great changes have taken place since you left.
Considerable time has elapsed since we have been here.
4) It is +一段时间+ since从句
It is two years since I became a postgraduate student.
延续动词表示经验、经历； 瞬间动词表示行为的结 果，不能与表示段的时间状语连用。
He has completed the work. 他已完成了那项工作。 (表结果)
I've known him since then. 我从那时起就认识他了。(表经历)
2) 用于till / until从句的差异
He didn't come back until ten o'clock.
He slept until ten o'clock.
1. You don't need to describe her. I ___ her several times.
A. had met B. have met C. met D. meet
答案B. 首先本题后句强调对现在的影响，我知道她的模样，你不用描述。再次，several times告知为反复发生的动作，因此用现在完成时。
2.---I'm sorry to keep you waiting.
---Oh, not at all. I ___ here only a few minutes.
A. have been B. had been C. was D. will be