英语中强调句和形式主语有什么差别，形式主语和定从的关系？

强调句型是对句子的主语，宾语，状语等进行强调，（谓语除外），其基本结构是It is +被强调的部分+that/who+句子的其他成分（强调主语若是人引导词可以用who/that,若是其他一律用that ）

比如：I lost my purse at the sreet.如果强调主语"I”，套用句型:It is I that/wholost my purse at the street.如果要强调地点，同样套用句型：It is at the street thatI lost my purse.通过这2个例子，我们可以知道 It is ...that是加进来的，所以判断一个句子是否是强调句，就是把It is ...that去掉，如果去掉之，还是一个完整的句子就是强调句。比如上面那个例子：It is at the street thatI lost my purse.把it is和that去掉还剩下at the street I lost my purse.这还是一个完整的句子。下面出2个句子，你自己判断一下：

It was in the year 1925 that John found a way to send pictures by wireless.

was it in the place that the accident happened last night ?

去掉it is 和that 这2个句子变成了：in the year 1925 John found a way to send pictures by wireless.

in the place the accident happened last night ？这2句句子结构完整，所以都是强调句。

It 做形式主语：有时候句子的主语过长，这样会引起头重脚轻，我们就用it做形式上的主语，而把真正的主语放在后面。比如：To learn English is difficult. 我们可以把To learn English 用it来代换做形式上的主语，把真正的主语To learn English 放在后面，就变成了：It is difficult to learn English.

It 做形式主语常用于以下句型：It is +adj./n (for sb)to do sth.

It is no use/good in doing sth.等等

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