最常見的是名詞和代詞做主語 (不定式+動名詞也可做主語)

It is safer to live in the country . 住在鄉下較安全。

初學英文的人容易將這句中文譯成：

Live in the country is safer. （錯誤）

這肯定是錯誤的句子，因為動詞不能直接做主語。動詞應變成不定式（即to+動詞原形）或動名詞（即動詞詞尾+ ing）方可作主語。

故以上例句應變成：

To live in the country is safer.

=Liveing in the country is safer.

但這種不定式或動名詞短語所形成的主語往往過大，故寫作的人經常用 it 代替這些過大的主語，不定式或者動名詞短語則移至句尾。

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.

了解上面的說明後,我們不妨試譯下面的句子:

中文:學英文很有趣.

英文: Learn English is interesting.(錯)

It is interesting to learn English.(對)

It is interesting learning English.((對))