這是強調句型,強調when
(1) 陳述句的強調句型：It is/ was + 被強調部分（通常是主語、賓語或狀語）+ that/ who（當強調主語且主語指人）+ 其它部分.強調句可以強調除謂語以外的一切成分.下面我們針對I met Li Ming at the railway station yesterday.句子進行強調.
強調主語：It was I that (who) met Li Ming at the railway station yesterday.
強調賓語：It was Li Ming that I met at the railway station yesterday.
強調地點狀語：It was at the railway station that I met Li Ming yesterday.
強調時間狀語：It was yesterday that I met Li Ming at the railway station.
(2) 一般疑問句的強調句型：同上,只是把is/ was提到it前面.
e.g.Was it yesterday that he met my old friend?
(3) 特殊疑問句的強調句型：被強調部分（通常是疑問代詞或疑問副詞）+ is/ was + it + that/ who + 其它部分?
e.g.When was it that you were born?
我們需要注意的是：第一、構成強調句的it本身沒有詞義；強調句中的連接詞一般只用that,who,即使在強調時間狀語和地點狀語時也如此,that,who不可省略.
第二、It's .that/who.的強調句型只是為了強調某一成分,而不充當成分.所以它和It作形式主語,真正的主語從句后置的情況有所不同.
我們一起看下面兩個例句:
1.It's necessary that we should learn English.(主語從句)
2.It's not until he got off the bus that he realized his money was stolen.(強調句型)
解析：因為強調句型中的It's.that/who.在句子中不充當任何成分.所以去掉后句子依然完整.
例子2去掉It's.that/who.后就成了
until he got off the bus he realized his money was stolen.而例子1去掉后句子則不完整.
(4) not … until … 句型的強調句
1、句型為：It is/ was not until + 被強調部分 + that + 其它部分
e.g.普通句：He didn’t go to bed until/ till his son came back.
強調句：It was not until his son came back that he went to bed.