英语时态讲解及练习,急求.

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现在完成时 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
共同的时间状语:
this morning, tonight,this April, now, once,before, already, recently,lately
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,不确定的时间状语
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e, go, leave, start, die, finish, bee, 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.）
He has been in the League for three years.
(在团内的状态可延续)
He has been a League member for three years.
(是团员的状态可持续)
He joined the League three years ago.
( 三年前入团,joined为短暂行为.)
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 I've (ever) seen.
这是我看过的最好的电影.
This is the first time (that) I've heard him sing.这是我第一次听他唱歌.
注意：非延续性动词的否定形式可以与表示延续时间的状语连用.即动作不发生的状态是可以持续的.
（错）I have received his letter for a month.
（对）I haven't received his letter for almost a month.
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