'过去完成时典型例题 The students ___ busily when Miss Brown went to get a book she ___ in the office. A. had written, left B，were writing, has left C. had written, had left D. were writing, had ...'

'过去完成时 1） 概念：表示过去的过去其构成是had +过去分词构成。那时以前 那时 现在 2） 用法a. 在told, said, knew, heard, thought等动词后的宾语从句。She said (that) she had never been to Paris.b....'

'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+ agoI have been here since five months ago....'

'比较since和forSince 用来说明动作起始时间，for用来说明动作延续时间长度。I have lived here for more than twenty years.I have lived here since I was born..My aunt has worked in a clinic since 1949.S...'

'用于现在完成时的句型 1）It is the first/second time…. that…结构中的从句部分，用现在完成时。It is the first time that I have visited the city.It was the third time that the boy had been late. ...'

'比较过去时与现在完成时 现在完成时可表示持续到现在的动作或状态，动词一般是延续性的，如live, teach, learn, work, study, know. 过去时常用的非持续性动词有come, go, leave, start, die, finish, become...'