当动词不定式作主语时,有哪些情况用it作形式主语?

一般情况下不定式作主语都要用it作形式主语来代替的.
Ⅰ.动名词和不定式做主语的相同点
1.不定式短语和动名词做主语时,常常为了保持句子的平衡,用it来做形式主语,如：
It is not easy to learn English well.
It is no use asking him for help.
2.如果主语用了不定式,表语和宾语也要用不定式;主语用了动名词,表语和宾语也要用动名词,这在英语中称为对称性,如:
To see is to believe.= Seeing is believing.(眼见为实)
(85)It is better to stay home than _A__ out.
A.to go B.going C.go D.gone
Ⅱ.动名词和不定式做主语的不同点
1.不定式做主语常表示具体的某一动作;表示泛指或一般的抽象的概念时,多用动名词,如:
(92)_B_ is a good form of exercise for both young and old.
A.The walk B.Walking C.To walk D.Walk
2.在”There/It +be+ no use(good/help/need)”之后常用动名词做主语,而少用不定式,如:
It /There is no use crying over the split milk.
It /There is no help taking this medicine.
★但是如果该句型中用了for引出不定式的逻辑主语,就只能用不定式,如:
(93) There is no good for us to do that exercise.
3.不定式可以和when,where,how,what,whether等连用,在句子中做主语,宾语和表语,而动名词则不能,如:
How to solve the problem is a hot potato.
Where to put the box seems hard to decide.
Whether to do it hasn’t been decided.
What to do next is up to you.那请问不定式作主语为什么要用it作形式主语来代替呢？？说实话，这个我还真不知道！我觉得应该是跟it的作用有关系吧！一般情况下不明的东西都可以用it来说明。比如说打电话的时候会说：who is this speaking? 回答者就可以说：it is john 等等！你问我的这个问题就相当于这样的！为什么在英语中“他”要翻译成He呢？希望你可以理解我的回答！