真正的主詞是以that為首的名詞子句，置於句後，而把形式主詞it置於句首，這在英文中極為常用。
(a) It is certain that he will get well in a day or two.(b) It is a fact that there are many ups and downs in our lives.
certain一定的；get will復元；in a day or two一兩天內；ups and downs榮枯，盛衰，沉浮。
(a) 他在一兩天內一定會復元。(b) 我們的生活的確是屢沉屢浮。
這種用法並非只限於that-clause，whether, if等連接詞，以及what, who, how等疑問詞所引導的名詞子句也可以作為真主詞。另外，(a) = He is certain to get well in a day or two.
1. It is said that he died of cancer. (據說他死於癌症。)2. It is doubtful whether he’ll succeed or not. (他能否成功難以確定。)3. It is not clear which is better. (哪個比較好並不清楚。)
〈注〉1. 若寫成單句，= He is said to have died of cancer.。請注意句中用的是完成式的不定詞。
It matters little who does it so long as it is done. (只要有做，誰做的並不重要。)