1.  不定詞可以當句子的主詞，如「To eat breakfast is good to our health.」(吃早餐有益身體健康。)

2.  但是英文裡有「假主詞－it」，可以放在句子開頭，用來代替真正的主詞，而真正的主詞則放到最後。

3.   因為英文的句子比較習慣「頭輕腳重」，尤其是主詞很長的句子，一定都把真主詞擺到最後，用「假主詞it」開頭。

 e.g. 1It is + 形容詞／名詞 + (for人) + to + 原形動詞

It is not convenient for me to take a bus to school. (我搭公車到學校不方便。)= To take a bus to school is not convenient for me.

※（it = take a bus to school）  (╳) I am not convenient to take a bus to school.

※    這裡的形容詞不是用來形容人，是用來形容事物的，所以不能以人當主詞後面加形容詞。

 e.g, 2It is + 形容詞 + (of人) + to + 原形動詞

It is so rude of you to come in without knocking on the door. (你沒敲門就進來真是沒禮貌。)

= To come in without knocking on the door is so rude of you. (不常用)

※ （it = to come in without knocking on the door）= You are so rude to come in without knocking the door.

※   這裡的形容詞都是用來形容人，所以能夠以人當主詞，後面加形容詞。

＊    補充說明：「for」與「of」後面加受格  「for」與「of」都是介系詞，所以後面的代名詞都要用受格：

(○)It is good for him to take part in the activity.    (╳)It is good for he to take part in the activity.