有時(shí)為避免句子頭重腳輕，常用形式主語(yǔ)it代替主語(yǔ)從句作形式主語(yǔ)放于句首，而把主語(yǔ)從句置于句末。主語(yǔ)從句后的謂語(yǔ)動(dòng)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(shí)
It is a surprise that 令人驚奇的是
It is a fact that 事實(shí)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(dòng)詞的過(guò)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(dòng)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(yǔ)從句中用來(lái)表示驚奇、不相信、惋惜、理應(yīng)如此等語(yǔ)氣時(shí)，謂語(yǔ)動(dòng)詞要用虛擬語(yǔ)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

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