动名词和不定式做主语时，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.
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)
当动名词做主语时，通常表示泛指或一般的抽象概念，例如：Walking is a good form of exercise for both young and old. 而不定式则常表示具体的某一动作，如：To walk is a good form of exercise for both young and old.
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，例如：There is no use crying over the split milk. There is no help taking this medicine. 但如果该句型中用了for引出不定式的逻辑主语，就只能用不定式，如：There is no good for us to do that exercise.
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.
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