一、什麼是動詞不定式呢？
1.動詞不定式的基本形式是「to+動詞原形」，有時可以不帶to。
2.不定式在句中不能單獨作謂語，是一種非謂語動詞結構。
3.不定式保留動詞的某些特性，可以有自己的賓語、狀語等。
4.動詞不定式可以作句子的主語、表語、賓語、定語、補語、狀語。
動詞不定式例子：
to live in Fuzhou
to read a book
to talk with the boy
to learn English well
二、不定式作主語
To learn English wellis very important. 學好英語很重要。
To try our best to deal with these problems is our duty.
To climb the mountainis very difficult for us.對我們來說爬這座山很困難
To get to the bus stationtakes me two hours.
花費我兩個小時到達公交車站。
To seeis to believe. 眼見為實
How to learn English wellis important. 如何學好英語很重要。
1. 動詞不定式作主語，謂語動詞用第三人稱單數形式。
2.直接把動詞不定式置於句首的情況不多。多數情況下，如果作主語的動詞不定式太長，為了保持句子的平衡(避免句子頭重腳輕)，常用it作形式主語，代替真正的主語──動詞不定式。而真正的主語──動詞不定式，則放在句子的末尾。
It is very important to learn English well
形式主語真正的主語
It is our duty to try our best to deal with these problems
形式主語 真正的主語
It is very difficult for us to climb the mountain
形式主語 真正的主語
It is very kind of you to help me with English
形式主語真正的主語
It takes me two hours to get to the bus station.
形式主語真正的主語
備註：辨析句型
It's + adj. + of sb.+ to do sth. 與 It's + adj. + for sb.+ to do sth.
一般來說，of sb.前面的形容詞是careful, careless,clever, foolish, polite,impolite,lazy,good, kind, nice, wise等，主要說明動詞不定式的行為人的性格、品質；for sb.前面的形容詞是dangerous, difficult, easy, hard, heavy, important, interesting, necessary等，一般說明不定式動作的特性。
It's very kind of you to help me. 你能幫助我，真是太好了。
It's foolish of him to make such mistakes.他真傻啊，竟然犯這種錯誤。
It's dangerous for you to swim alone.你自己一個人去遊泳很危險。
It's very difficult for us to climb the mountain.對我們來說爬這座山很困難。
還有一個區別是：在of sb. 中，可用sb.直接作主語來改寫句子。而for sb.結構是不可以這樣轉換的。
It's very kind of you to help me. = You are very kind to help me.
練習：
1. Is ______ necessary to return the book tomorrow ?
A. this B. that C. it D. which
2. It’s necessary ____ us ____ wear suitable clothes on every occasion.
A. for; to B. for; for C. of ; to D. to; for
3.It is very kind_____you to help me with my English.
It's hard____ me to learn it well.
A. for, of B. of, of C. of, for D. for , for
4.It's important______the piano well.
A. for him to play B. of him playing
C. of him to play D. for him playing
5.It is difficult______us to finish this work in an hour.
A. for B. of C. with D. to
6.It's clever_____you to work out the math problem.
A. for B. of C. with D. to
參考答案：
1.C ， 2.A，3.C，4.A，5.A,6.B