it 是形式主语， that引导的是主语从句，整体代表真正的主语；
属于强调句型。
(1) 陈述句的强调句型：It is/ was + 被强调部分（通常是主语、宾语或状语）+ that/ who（当强调主语且主语指人）+ 其它部分.强调句可以强调除谓语以外的一切成分.下面我们针对I met Li Ming at the railway station yesterday.句子进行强调.
强调主语：It was I that (who) met Li Ming at the railway station yesterday.
强调宾语：It was Li Ming that I met at the railway station yesterday.
强调地点状语：It was at the railway station that I met Li Ming yesterday.
强调时间状语：It was yesterday that I met Li Ming at the railway station.
(2) 一般疑问句的强调句型：同上,只是把is/ was提到it前面.
e.g.Was it yesterday that he met my old friend?
(3) 特殊疑问句的强调句型：被强调部分（通常是疑问代词或疑问副词）+ is/ was + it + that/ who + 其它部分?
e.g.When was it that you were born?
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