除祈使句 (imperatives) 外，英文的子句都要有主詞。有時當動詞沒有主詞，因為真正的主詞位在句中某處時，我們必須使用形式主詞或假主詞 (preparatory subjects or anticipatory subjects or dummy subjects)。英文所用的假主詞有二：There 和 it。
There 在 There is (There’s) 和 There are 結構中是當作假主詞，真正主詞是 There is 或 There are 後面的名詞或名詞片語。例如：
- There’s a girl on the phone for you. (有個女生打電話找你) (正) － 這句的 There 是假主詞，真正主詞是 (a) girl。
She’s a girl on the phone for you. (誤)
It’s a girl on the phone for you. (誤)
Is a girl on the phone for you. (誤) - There are three foreigners in our community. (我們社區有三個老外) (正) － 這句的 There 是假主詞，真正主詞是 (three) foreigners。
They are three foreigners in our community. (誤)
Are three foreigners in our community. (誤)
It 在句中當假主詞時，真正主詞有三種型態：不定詞片語、動名詞片語及 that 子句。例如：
- It’s very hard to believe that he is an honest employee. (很難相信他是個誠實的員工) (正)
Is very hard to believe that he is an honest employee. (誤)
－ 這句的 It 是假主詞，真正主詞是不定詞片語 “to believe that he is an honest employee”。所以，這句可改成 To believe that he is an honest employee is very hard.。 - It’s worth/worthwhile calling him to clear up the misunderstanding between you and him. (值得打電話給他澄清你和他之間的誤會) (正)
Is worth/worthwhile calling him to clear up the misunderstanding between you and him. (誤)
－ 這句的 It 是假主詞，真正主詞是動名詞片語 “calling him to clear up the misunderstanding between you and him”。所以，這句可改成 Calling him to clear up the misunderstanding between you and him is worthwhile.。 - It is difficult (that) they would finish their work in two days. (他們要在兩天內完成工作是有困難的) (正)
Is difficult (that) they would finish their work in two days. (誤)
－這句的 It 是假主詞，真正主詞是名詞子句 “that they would finish their work in two days”。所以，這句可改成 That they would finish their work in two days is difficult.。注意：若將真正主詞 (即 that 所引導的名詞子句) 置於主句的動詞之前，則 that 不能省略。