1.当it为形式主语时，to do/doing常可互用, 但to do 形式更为常用。
It’s difficult to make the air clean.
It’s difficult making the air clean.
2.但当表语是necessary, important等形容词时，后面只用不定式。而表语是no use, no good, a
waste of time时，只用动名词。
It is necessary to spend enough time on English if you want to learn it well.
It is no use at all arguing with him about it. 这件事和他争论根本没用。
