1.to do 可以做主语
To see is to believe.
眼见为实。
To walk is good for health.
走路有利健康。
To have good health is extremely important.
身体健康是极其重要的。
2.不定式作主语较正式，常用it作形式主语, 结构常为：
It is + adj. +（for/of sb.）+ to do sth.
It is useful to ask your mother for some advice.
征求你母亲的意见是有用的。
（真正主语to ask your mother for some advice，形式主语it）
It is wrong to be rude.
没礼貌是不对的。
It is good to love many things.
热爱很多事物是好的。
It is easy for me to learn English.
对于我来说，学习英语是容易的。（easy是表示客观情况，所以me前用for）
It is clever of you to do so.
你这样做是聪明的。（clever是表示人本身的特点，所以you前用of）
It is + n. + to do sth.
It is your duty to take care of your baby.
照顾好你的小孩是你的责任。
It is my job to make sure you have everything you need.
确保满足你的一切需要是我的职责。
It is my honor to be invited to the wedding.
受邀参加婚礼是我的荣幸。
It takes (sb.) + 时间 + to do sth.
It took me two hours to finish the homework.
完成作业花了我两小时。
It takes four years to finish college education.
完成大学教育需要四年。
It will take you four hours to get to Florida.
到弗罗里达将花费你四小时。
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