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 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)
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)
To finish the task will take a long time. 要完成這項任務(wù)將要花費很長時間。（具體）
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.