题目内容：

It be+ ...since/before/when 分别在哪些情况下用?

优质解答

一before.
1.It was/will be+时间段+before... 过了…才…
eg:It was three years before he finished the work.他花了三年才完成这项工作.
It will be three years before he finishes the work.他得花三年才能完成这项工作.
2.It won't be/take long before...不久就会…
eg:It won't be long before he finishes the work.他不久就会完成这项工作.
二since.
1.It is/has been+时间段+since...自从…以来,过了…
eg:It is/has been three years since he finished the work.自从他完成这项工作已经三年了.
2.该句型中若since后跟延续性动词,要翻译成否定含义,即“没做某事已经多久了”.
eg:It has been three years since he worked here.他不在这工作已经三年了.
It has been three years since he smoked.他已经戒烟三年了.
3.若主句非It is…形式,则主句还可用过去完成时.
eg:He had been worrying since the letter arrived.自从接到那封信后他就一直焦虑不安.
三when.
when没有固定与It is…连用的句型.when可表示时间点或时间段,因此从句中短暂性动词或延续性动词均可用.when还可作并列连词,表示突然发生一个动作,等于and at that time.常用句型:
1主语+was/were doing when...
2主语+was/were about to do when...
3主语+had/hadn't done for some time when...
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