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介词短语作时间状语,直接关系到时态。下面我们探讨几个作时间状语的介词短语的意义及其与时态的关系。
一、by+时间
by+时间表示的是一个含糊的时间概念。它并不精确地表明动作发生的时间。具体地说,它有以下两种意义:
1.到……为止
这一意义我们可以理解为在……之前,某一动作已经完成。有这一意义的短语作状语,句子的谓语动词通常用完成时。由于by后面可加三种时间,句子的谓语可有三种不同的完成时态。
(1)by+this week(month...)到这一周(月……)为止,表明动作的发生和说话的时间处于同一段时间内。有这样的介词短语作状语,句子的谓语动词常用现在完成时。
We have received five thousand visitors by this week.
(2)by+过去时间,表明动作到过去某一点为止已经完成。有这样的介词短语作状语,句子的谓语动词常用过去完成时。
I had finished all the exercises by nine o'clock yesterday.
(3)by+将来时间,表明动作到将来某一点为止即将完成。有这样介词短语作状语,句子的谓语动词常用将来完成时。
①I will have worked here for ten years by next July.
②I will have finished the exercises by nine o'clock.
2.by+时间构成的短语第二个意义是“最迟不超过……”
在这一意义上,by后可加两种时间。句子谓语动词的时态根据具体的语言环境也有所不同。
(1)句子阐述的是一般性动作,其谓语动词用一般现在时或一般过去时,by后所加的时间是通常的时间。例如:
①In those days when there was no TV or other entertainments,the Villagers usually went to bed by 8 o'clock.
②Nowadays children attend school by the age of eight.
有时也可以用进行时,其含义为:最迟不超过……。例如:
To become top gymnasts,...girls are already performing in compentitions by the age of ten,...
(2)by+将来时间,表示动作发生,最迟不超过……,有这一介词短语作状语,句子的谓语动词用将来时。
The train leaves at 6.I'll be at the station by 5:40 at the latest.
二、at+时间
at+时间表示的是一个精确的时间概念,它表明动作发生的精确时间。at后除可加通常时间,句子用一般现在时或一般过去时外,还可有下列三种情况:
1.at+过去的某一具体时间(时刻)作状语,句子的谓语动词如果是延续动词则用过去进行时:
I was watching TV at 8:50 yesterdsy.
2.at+将来时间(时刻)作状语,句子的谓语动词如果是延续性动词则用将来进行时。
I will be sitting in the train travelling at 8tomorrow.
如果句子的谓语动词为短暂性动词,则用一般将来时。但这时要注意at与by的区别。如上所述,at+时间表示的是一个精确的时间概念。有些情况下,语言环境决定了动作发生的时间并非那样精确,所以不宜用at,而用by,例如我们可以说:The meeting will begin at nine.但却最好说:These photos will be ready by nine.
三、before
before所组成的介词短语通常有如下两个意思:
1.在……之前,有这一意义的短语作状语,句子的谓语除可用一般现在时或一般将来时之外,如表发生过的动作还可用一般过去时、过去完成时或过去完成进行时。
①He prepared his lessons before supper yesterday.
②He had prepared his lessons for two hours before supper yestelday.
③He had been preparing his lessons all the time before supper yesterday.
2.……之后,有这一意义的短语作时间状语,句子通常是下列句式:
It will be+一段时间+before…
It will be two weeks before the final exam.两周之后我们将进行期末考试。
四、Since
since所组成的介词短语,其意思为:自从……以来。有这一短语作时间状语,句子的谓语动词可根据语境用完成时,或者用完成进行时。
①He has lived in the village since the end of last year.
②I've been reading books since 10 o'clock,and perhaps l should have a rest.
除上述情况外也有下列句式:
It is/has been+一段时间since...或
It was/had been+一段时间since...
①It is/has been ten years since her graduation from college.
②It was/had been 15 years since her absence from home.
五、in与after
1.in后通常加一段时间,如:in three days或in three days'time。其意思根据具体语言环境而定。如果用于一般过去时的句子作状语,其意思为“多久之内”。例如:
The operation was over in half an hour.
如果用于将来时的句子作状语,其意思为“多久之后”。例如:
I suppose these ideas will be turned into realities in five years.
2.after后除可以加时间点外(如:after Sunday),也可加一段时间,但after加一段时间(如:after three days)通常用于一般过去时的句子作状语,不作将来时句子的状语。例如我们只可说:He'll be back in three hours.而不说He will be back after three hours.