当从句中的主语为动词不定式时，常用it做形式主语，如：
1）I think it is important for me to learn English well .
=I think it important for me to learn English well .
2）I find it is interesting for me to learn English.
=I find it interesting for me to learn English.
主语太长，句子会头重脚轻时，如：
That he made such a mistake is a pity .
It is a pity that he made such a mistake.
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