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.
11.14 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。