How do you study for a test?现在完成时定义现在完成时用来表示之前已发生或完成的动作或状态，其结果的确和现在有联系。动作或状态发生在过去但它的影响现在还存在；也可表示持续到现在的动作或状态。其构成： have (has) +过去分词。 比较过去时与现在完成时 1）过去时表示过去某时发生的动作或单纯叙述过去的事情，强调动作；现在完成时为过去发生的，强调过去的事情对现在的影响，强调的是影响。 2）过去时常与具体的时间状语连用，而现在完成时通常与模糊的时间状语连用，或无时间状语。 一般过去时的时间状语: yesterday, last week, ago, in1980, in October, just now, 具体的时间状语 现在完成时的时间状语 for, since, so far, ever, never, just, yet,till / until, up to now, in past years, always, 3）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, get married等。 举例： I saw this film yesterday.（强调看的动作发生过了。） I have seen this film. （强调对现在的影响，电影的内容已经知道了。） 句子中如有过去时的时间副词（如 yesterday, last, week, in 1960）时，不能使用现在完成时，要用过去时。 （错）Tom has written a letter to his parents last night. （对）Tom wrote a letter to his parents last night. 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. 2）This is the that结构，that 从句要用现在完成时. This is the best film that Ive (ever) seen. 这是我看过的最好的电影。 This is the first time (that) Ive heard him sing.这是我第一次听他唱歌。 比较since和for Since 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. Some new oilfields have been opened up since 1976. I have known Xiao Li since she was a little girl. My brother has been in the Youth League for two years. I have not heard from my uncle for a long time. 注意：并非有for 作为时间状语的句子都用现在完成时。 I worked here for more than twenty years. (我现在已不在这里工作。) I have worked here for many years. (现在我仍在这里工作。) 小窍门：当现在完成时+一段时间，这一结构中，我们用下面的公式转化，很容易就能排除非延续动词在完成时中的误使。 1)(对) Tom has studied Russian for three years. = Tom began to study Russian three years ago, and is still studying it now. 2)(错) Harry has got married for six years. = Harry began to get married six years ago, and is still getting married now. 显然，第二句不对，它应改为 Harry got married six years ago.或 Harry has been married for six years. 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. 4) It is +一段时间+ since从句 It is two years since I became a postgraduate student.