一、it作不定式的形式主語

a) it + be + 形容詞 + 不定式

It is difficult to get there before dark. 天黑以前趕到那裡很困難。

It is better to get up early. 早起會更好。

b) it + be + 名詞 + 不定式

It was his duty to attend to this matter. 處理這個問題是他的責任。

It is the file system's job to manage writing data between the disks and memory. 在記憶體和磁碟上管理資料是檔案系統要處理的東西。

c) it + 動詞 + 賓語 + 不定式

It takes a lot of effort to recover the system. 恢復系統不是一件容易的事情。

It makes me sad to hear that you have to go away. 聽說你一定要走，我很難過。

d) it + be + 介詞短語 + 不定式

It is against the law to steal. 偷竊是犯法的。

二、 it作動名詞的形式主語

a) it + be + 名詞 + 動名詞 (常用的名詞有no good, no use, nuisance, a waste, a wonder 等)

It’s a wonder meeting you here! 在這裡遇見你真是個奇蹟！

It’s no use doing a lot of work without thoughtful planning. 沒有考慮周到的計劃而做一大堆工作是沒用的。

b) it + be + 形容詞 + 動名詞

It was very difficult getting everything ready in time. 要及時做好一切準備很難。

Is it worthwhile hating each other over such a trifle? 為了這麼點事兒，你倆至於結冤嗎？

c) 其他情況

It doesn’t matter putting these two files together. 把這兩份檔案放在一起沒關係。

It doesn’t make any difference my being here. 我是否在這兒並不能改變什麼。（注:這裡my為動名詞的邏輯主語）

三、 it作主語從句的形式主語

a) it + be + 形容詞 + 從句

It is clear that the chosen pattern is not the correct one. 顯然，所選擇的圖式是錯誤的。

It is doubtful whether this method is more scientific. 這個方法是否更科學值得懷疑。

b) it + be + 名詞 + 從句

It is a pity that we shall have to leave tomorrow. 我們明天必須離開，真是遺憾。

It is a complete mystery what caused the accident. 是什麼引起這場事故完全是個謎。

c) it + 動詞 (+賓語或狀語) + 從句

It matters little who does it so long as it is done. 只要事情做了，誰做都行。

It happens that his sister is a closed friend of mine. 碰巧她姐姐是我的一個好朋友。

d) it + 動詞的被動語態 + 從句

It is said that the rhythm of tides is affected by the moon. 據說潮汐的漲落受月亮的影響。

It is estimated that the output will increase by 20% this year. 今年預計產量將提高20%。

e) it + 介詞 + 從句

It is out of question that economy will keep growing for 40 years. 經濟增長40年應該不成問題。

主語從句放在句首，句子常常顯得比較笨重，因此常把它移到句子末尾，而用It來作形式上的主語，例如It is a pity that Brown can't attend our English meeting,真可惜布朗不能出席我們的英語晚會，It is stange that he did ont come yesterdy很奇怪，他昨天沒有來。