It做形式主语时其后 to do 和 ing 的不同用法 及具体例句

动词不定式和动名词都属于动词的非限定形式，它们既有动词的性质，又有名词的性质。动词不定式有两种形式：一是带to的不定式，一是不带to的不定式，后者即通常所谓的动词原形；动名词则由动词原形加上词尾-ing构成，与现在分词的构成法是一样的，它们都有时式和语态的变化。动词不定式的一般式是to do和to be done，完成式是to have done和to have been done，进行式是to be doing和to be being done，完成进行式是to have been doing和to have been being done；动名词的一般式是doing和being done，完成式是having done和having been done。
动词不定式在句中可以作主语，如：
To err is human, to forgive devine.
To see is to believe.
To do that implies taking responsibility.
当代英语常常用it作形式主语，把动词不定式放在谓语之后。
It takes ten people to encircle the big tree,
How long did it take you to finish the work?
动名词在句子中也可以作主语，其逻辑主语泛指人们：
Seeing is believing.
Saving is having,
it作形式主语，把真正的主语——动名词放在句末的情况比较少，常用的结构有：it is后接no use，no good、fun等名词，如：
It is no use crying over spilt milk.
It is no good objecting.
It is great fun sailing a boat.
it is后还可接useless、nice、good、interesting等形容词。如：
It is useless speaking.
It is so nice sitting here with you.
It is good hearing English spoken.
It作形式主语时，位于谓语之后作真实主语的非谓语动词以不定式为多，除了在上述名词和形容词后面用动名词以外，一般都用动词不定式作真实主语。希望我能帮助你解疑释惑。

it做形式主语后面通常情况是跟to do.没怎么看到用doing 的。例如：It's+adj. for/of sb. to do sth.
It's important for you to study English well. It's kind of you to help me.

it is +noun for sb to do sth
for example, it is time for the government to take some measures