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(yīng)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

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