凡是省略了to的不定式作賓語補足語，變被動語態時to都要還原。常見的有：

1、使役動詞 make （let, have ）sb do sth

2、感官動詞 see （hear ,watch ,listen to , notice ）sb do sth 例如：1) .We often see him play football . He is often seen to play football by us .2). The boss made his workers work 12 hours a day . The workers were made to work 12 hours a day by the boss .3) I heard her sing in the room yesterday . She was heard to sing in the room by me yesterday .

因為主動句的時候看似是謂語動詞+動詞原形,實際這裡不是動詞原形,而是省略了to的動詞不定式.

舉例：make sb do sth,這裡看似是加上的動詞原形,實際是make sb to do sth,只是省略了這個to,而且主動句中必須省略,所以變成被動句才多了一個to

但是,請注意不是所有的加動詞原形的變成被動都要還原to,一般是使役動詞（翻譯為“讓”）和感官動詞（翻譯為“...起來”）

如：She bade me (to) come in。 她叫我進來。

He bade me (to) do the work。
他吩咐我去做那項工作。

但是，在當bid為被動語態時，其後的不定式必須帶to。如：I was bidden to do the work。 我被囑咐去做那項工作。

變被動語態要還原動詞不定式符號to，即要帶上to.

例：

主動：

My mom lets me help her with the housework.

變被動為：I am let to help my mom with housework.

Let的過去式，過去分詞都是原形

因為在主動語態時，就省略了非謂語的不定時to，make sb do為例，被動語態sb be made to do是，to應該加上，否則會產生歧義。