1）不定式作主语一般表示具体的某次动作.
To complete the 30 storied building in one year was quite a difficult task.
2）动名词作主语一般表示泛指意义的行为.(这时也可用动词不定式)
Seeing is believing.眼见为实.
注意：
1）当主语较长,谓语较短时,常用it做形式主语,而将不定式放到谓语的后面.
It is important for modern young people to master at least two foreign languages.
2）在It’s no use(或good, need)…或It’s useless（或worthwhile）…等结构中,常用动名词作真正的主语：It is no use sending him over.派他去没用.
3）在There is no +v.-ing…结构中,常用动名词作主语：
There is no joking about the matter.这事开不得玩笑.
4）比较状语从句里的主语用动词不定式还是用动名词,取决于主句里主语的形式：
It’s easier to read a foreign language than to speak it.
Reading a foreign language is easier than speaking it.阅读外语比讲外语容易.