4.6 since 的四种用法
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 college student.
4.7 延续动词与瞬间动词
1) 用于完成时的区别 
    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
延续动词用于肯定句，表示 " 做 …… 直到 ……" 瞬间动词用于否定句，表示 " 到 …… ，才 ……"
He didn't come back until ten o'clock.  他到 10 点才回来。
He slept until ten o'clock.  他一直睡到 10 点。
典型例题
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
答案 A. 等待的动作由过去开始，持续到现在，应用现在完成时。