1）動名詞作主語通常表示抽象動作；而不定式作主語表示具體動作.
Smoking is prohibited(禁止)here．這里禁止抽煙.(抽象)
It is not very good for you to smoke so much．你抽這么多煙對你身體很不好.(具體)
（2）動名詞作主語時,通常用以表示一件已知的事或經(jīng)驗.
不定式短語通常用來表示一件未完成的事或目的.
Climbing mountains is interesting．爬山很有趣.(經(jīng)驗)
Driving a car during the rush hour is tiring．在高峰時刻開車令人厭煩.(經(jīng)驗)
（3）不定式做主語,一般用it當(dāng)形式主語,把作主語的不定式短語后置.
It took me only five minutes to finish the job.
2．不定式、動名詞和分詞作表語的區(qū)別
（1）不定式作表語
1)不定式作表語一般表示具體動作,特別是表示將來的動作.
To do two things at a time is to do neither．--次做兩件事等于未做.
What I would suggest is to start work at once.我的建議是立刻開始干.
2）如果主語是不定式（表示條件）,表語也是不定式（表示結(jié)果）.
To see is to believe.百聞不如一見.
To work means to earn a living．工作就是為了生活.