when was it that your mother showed you around the west lake语法结构,为什么用that

这是强调句型,强调when
(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?
我们需要注意的是：第一、构成强调句的it本身没有词义；强调句中的连接词一般只用that,who,即使在强调时间状语和地点状语时也如此,that,who不可省略.
第二、It's .that/who.的强调句型只是为了强调某一成分,而不充当成分.所以它和It作形式主语,真正的主语从句后置的情况有所不同.
我们一起看下面两个例句:
1.It's necessary that we should learn English.(主语从句)
2.It's not until he got off the bus that he realized his money was stolen.(强调句型)
解析：因为强调句型中的It's.that/who.在句子中不充当任何成分.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去掉后句子则不完整.
(4) not … until … 句型的强调句
1、句型为：It is/ was not until + 被强调部分 + that + 其它部分
e.g.普通句：He didn’t go to bed until/ till his son came back.
强调句：It was not until his son came back that he went to bed.