英语四级语法辅导-动词的时态7

　　11.13 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 postgraduate student.
　　11.14 延续动词与瞬间动词www.Examw.com
　　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