Question

I think that it is important to master a foreign language.(改为同义句)

I think _____ _____ ______ _______ a foreign language.
答案是it important to master, 可是还是不明白为什么，这涉及到什么形式主语、形式宾语之类的，我不太明白，可以帮忙解释一下吗？非常感谢~

Answer 1

作为形式主语或宾语的it并无实际语义，只是为满足语法上的需要，避免句子头重脚轻。原句I think that it is important to master a foreign language中，i是主语，think是谓语动词，think 后面接了一个that从句作宾语，句子的结构为主+谓+宾。that从句中，it作形式主语，真正的主语是后面的不定式短语to master a foreign language，还原主语就是to master a foreign language is important，is 是系动词，important是表语。同义句I think it important to master a foreign language中i还是主语，think还是谓语动词，只不过这时think有两个宾语，即双宾语（宾语+宾语补足语）。其中it作形式宾语，真正的宾语是后面的不定式短语to master a foreign language，important这时作为宾语补足语，补充说明宾语（to master a foreign language）的情况，这时还原就是I think to master a foreign language important。这时候句子的结构为主+谓+宾+宾补。

Answer 2

原句I think that it is important to master a foreign language.的it是形式主语。
I think it important to master a foreign language. it是形式宾语
1.it做形式主语
It is +adj. +(for sb.) +to do sth.
It is time to do/for/that....
2.it做形式宾语
主语+found/think+it +宾补（动词不定式、形容词等）具体可参考：http://attach.etiantian.com/staticpages/study/question/question_5007425.htm

Answer 3

应该用it
important
to
master
it在这里是形式宾语
因为它在动词think（谓语）后面
所以不是形式主语
因为it在这里是形式宾语
所以
如果在后面加is
那句子中就会有两个谓语
即think和is
这在英语中是不允许的
故
应用it
important
to
master

Answer 4

形式主语是指用it来代替一个比较长的主语（可以是主语从句、不定式等），避免头重脚轻。
如: To learn a second language is important. = it is important to learn a second language.
形式宾语是指用it来代替一个比较长的宾语（可以是宾语从句、不定式等），避免头重脚轻。
如：I find singing interesting.
i fing to play a joke on others interesting. 用一个it来代替“to play a joke on others”,然后把这个放在后面。变成：i fing it interesting to play a joke on others .
同样的：i think that it important to master a foreign language. it是形式宾语，真正的宾语是“to master a foreign language”
=i think that to master a foreing language important.

Answer 5

it 做形式宾语且前置，真正的宾语应该是介词词组yo master a foreign language