There came into his head what seemed a perfectly lovely idea.

句中 what 从句在句中的成分? 谢谢老师回答！

回答如下：

这个There 引导倒装结构，there 引导词，无词义，后面的主语从句为句子正真主语

其结构为There +谓语+ 其它+（主语从句）

句子正常语序为

What seemed a perfectly lovely idea came into his head .

这里连接代词what引导的主语从句可以转为话先行词+关系代词构成定语从句，请看下面

The thing that seemed a perfectly lovely idea came into his head.

【参考译文1】他突然想到一个似乎非常不错的一个想法.

【参考译文2】一个似乎完美的想法突然涌现在他的脑海里

句子语法结构分析如下

引导词:There 用来构成倒装句子结构，无意义

谓语：came into  突然想到；涌现出；浮现出

宾语：his head 他的脑海里；他的头脑里

主语从句：what seemed a perfectly lovely idea came into his head

一个似乎完美的想法出现他的脑海里/涌现在他的脑海里.

备注:There +谓语+名词/名词性从句 都属于倒装结构，但是如果主语为代词，则不需要倒装使用下面语序.这样使用还有here这个词.

There +代词+ 谓语

There it goes 

There they are .

Here it comes 

Here comes the bus

【关于what 引导名词性从句的五种特殊用法的语法拓展讲解如下】

【1】表示“……的东西或事情”，相当于“the thing that...; all that...; everything that....”：

They’ve done what they can to help her.

=They have done all that they can(do) to help her.

他们已经尽力帮助了她。

He saves what he earns.

他赚多少，积蓄多少。

 What Mary is is the secretary.

玛丽的职务是秘书。

【2】表示“……的人或的样子”，相当于the person that

He is no longer what he was.(表语从句）

=He is no longer the person that he was.

他已经不是以前的那个样子。

 He is what is generally called a traitor.

他就是人们通常所说的叛徒。

Who is it that has made Fred what he is now?

是谁把弗雷德培养成现在这个样子的？

【3】表示“……的数量或数目”,相当于the amount/number that

Our income is now double what it was ten years ago.

我们现在的收入是10年前的两倍。The number of the students in our school is ten times what it was before liberation.

现在我校学生的数量是解放前的10倍。

【4】表示“……的时间”,相当于the time that

After what seemed like hours he came out with a bitter smile.

似乎过了几个小时他才苦笑着出来。

The young girl was too frightened to speak, standing there for what seemed like hours.

小女孩吓得不敢说话，在那儿站了几个小时。

【5】表示“……的地方”,相当于the place that

This is what they call Salt Lake City.

这就是他们称为盐湖城的地方。

 In 1492, Columbus reached what is now called America.

1492年哥伦布到达了现在所称的美洲大陆。

【特别备注:用法说明】

 (1) 这样用的what相当于“先行词+关系代词”，所以其前不能有先行词：

你欢什么就拿什么吧。

正：You can have what you like.

正：You can have everything (that) you like.

误：You can have everything what you like.

⑵ 有时what 可用作限定词，其后修饰一个名词

I gave him what books I had.

我把我所有的书都给他了。

(3) What 后的名词可以是复数名词或不可数名词，但通常不能是单数可数名词：

误：I gave him what book I had.He gave me what money he had about him.

他把身边带有的钱全给了我。

(4) What后接不可数名词时，有时可有little修饰。注意区别：what+不可数名词=所有的都，what little+不可数名词=虽少但全部。 

 I spent what time I had with her.

我所有的时间都是与她一起度过的。

 I spent what little time I had with her.

 我仅有的一点时间都是与她一起度过的。