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
        
        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。