有時為避免句子頭重腳輕，常用形式主語it代替主語從句作形式主語放于句首，而把主語從句置于句末。主語從句后的謂語動詞一般用單數(shù)形式。常用句型如下：?
（1） It + be + 名詞 + that從句
It is common knowledge that ? ? ?是常識
It is a surprise that ? ? ?令人驚奇的是?
It is a fact that ? ? ?事實是?
（2）It + be + 形容詞 + that從句
It is necessary that ? ? ?有必要?
It is important that ? ? ?重要的是?
It is obvious that ? ? ?很明顯?
（3）It + be + 動詞的過去分詞 + that從句?
It is believed that ? ? ?人們相信?
It is known to all that ? ? ?眾所周知?
It has been decided that ? ? ?已決定?
（4） It + 不及物動詞 + that 從句?
It appears that ? ? ?似乎?
It happens that ? ? ?碰巧
It occurred to me that ? ? 我突然想起
另注意：在主語從句中用來表示驚奇、不相信、惋惜、理應如此等語氣時，謂語動詞要用虛擬語氣“(should) +do”，常用的句型有：
It is necessary (important, natural, strange, etc.) that
It is suggested (requested, proposed, desired, etc.) that