辨析——It is/was +时间+when+...;It will be/was +时间+befro...;It is/has been +时间+since..;强调句式；定语从句。的区分+

It is/was +时间+when+...; 该句型中的 when 引导的是一个时间状语从句，主句中的 it 指时间，表语由具体的时间充当，常译为＂当．．．的时候，是．．．＂ 　　
It was 5 o’clock when he came here.

It is + 时间 + before …
这个句型和上面句型中时间都是时间段，如long、years、months、weeks、five days、three hours、twenty minutes等。不过主句的时态多用一般将来时或一般过去时。
It will be several years before we meet again.
我们要过好几年才能再见面。
It was not long before they set out for the front.
不久他们就出发去了前线。
It is/was + 时间 + since … 从……已多久了。
It is three years since I met him in Beijing.
It was a long time since I (had) lived in that small mountain village with these farmers.（注意两句中的时态）
It is five months since I arrived in New York.我到纽约已经五个月了。
It is five months since I was in New York.我离开纽约已经五个月了。
（注意：终止性动词表示从动作发生时算起；如果是状态动词或持续性动词，则从动作或状态结束时算起。）
It is + 被强调部分 + that/who + …
Was it in the street that you met her?
Who was it that called him“comrade”?
It was not until yesterday that I met my old friend.
It is in a small factory that my brother is going to work.（注意上述各句均为强调句型的“考点”）
比较：It is a small factory where my brother is going to work.（定语从句）