比如：To have a balanced diet is very important. 這個就是動詞不定式作主語。 祝你學習進步，更上一層樓！(*^__^*) 有不會的可以再問我:）

不定式to do做主語的用法及例句，具體解析如下：

1.to do 可以做主語

To see is to believe.

眼見為實。

To walk is good for health.

走路有利健康。

To have good health is extremely important.

身體健康是極其重要的。

2.不定式作主語較正式，常用it作形式主語, 結構常為：

It is + adj. +（for/of sb.）+ to do sth.

It is useful to ask your mother for some advice.

徵求你母親的意見是有用的。

（真正主語to ask your mother for some advice，形式主語it）

It is wrong to be rude.

沒禮貌是不對的。

It is good to love many things.

熱愛很多事物是好的。

It is easy for me to learn English.

對於我來說，學習英語是容易的。（easy是表示客觀情況，所以me前用for）

It is clever of you to do so.

你這樣做是聰明的。（clever是表示人本身的特點，所以you前用of）

It is + n. + to do sth.

It is your duty to take care of your baby.

照顧好你的小孩是你的責任。

It is my job to make sure you have everything you need.

確保滿足你的一切需要是我的職責。

It is my honor to be invited to the wedding.

受邀參加婚禮是我的榮幸。

It takes (sb.) + 時間 + to do sth.

It took me two hours to finish the homework.

完成作業花了我兩小時。

It takes four years to finish college education.

完成大學教育需要四年。

It will take you four hours to get to Florida.

到弗羅裡達將花費你四小時。