It is safer to live in the country . 住在乡下较安全。
初学英文的人容易将这句中文译成：
Live in the country is safer. （错误）
这肯定是错误的句子，因为动词不能直接做主语。动词应变成不定式（即to+动词原形）或动名词（即动词词尾+ ing）方可作主语。
故以上例句应变成：
To live in the country is safer.
=Liveing in the country is safer.
但这种不定式或动名词短语所形成的主语往往过大，故写作的人经常用 it 代替这些过大的主语，不定式或者动名词短语则移至句尾。
To live in the country is safer.
=it is safer to live in the country.
Liveing in the country is safer.
=It is safer liveing in the country .
=It is safer to live in the country.
了解上面的说明后,我们不妨试译下面的句子:
中文:学英文很有趣.
英文: Learn English is interesting.(错)
It is interesting to learn English.(对)
It is interesting learning English.((对))