it✀s…… ，when……这是什么句式 谢谢

It's...when. It's....before等等都不是强调句
强调句

（一）强调句句型
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。
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？
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

强调句

（一）强调句句型
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。

e.g. It was yesterday that he met Li Ping.

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 Li Ping?

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 and where was it that you were born?

4、强调句例句：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.

5、注意：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；强调句中的时态只用两种，一般现在时和一般过去时。原句谓语动词是一般过去时、过去完成时和过去进行时，用It was … ，其余的时态用It is … 。

（二）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 wife came back.

强调句：It was not until his wife came back that he went to bed.

2、注意：此句型只用until，不用till。但如果不是强调句型，till, until可通用；因为句型中It is/ was not … 已经是否定句了，that后面的从句要用肯定句，切勿再用否定句了。

（三）谓语动词的强调

1、It is/ was … that … 结构不能强调谓语，如果需要强调谓语时，用助动词do/ does或did。

e.g. Do sit down. 务必请坐。

He did write to you last week. 上周他确实给你写了信。

Do be careful when you cross the street. 过马路时，务必（千万）要小心啊！

2、注意：此种强调只用do/ does和did ，没有别的形式；过去时用did ，后面的谓语动词用原形。

It be 时间段 before 从句 ：多久之后才....
It will be 一段时间before 一般现在 ：将来过多久才
It was 一段时间before 过去时 ：不久就...

it's... when 没什么特殊用法
给你造个句子吧
it's 3 o'clock when he gets home

中间有逗号吗？没有的话就是定语从句