被动语态还原to的口诀：一感二听三让四看半帮助。这些词包括feel，hear，listen to，make，have，let，see，watch，look at，observe，notice，help等。
被动语态中to的还原
不带to的动词不定式作宾语补足语的主动语态改为被动语态时，省略的to要加上。
常考的动词有make，let，have，see，watch，notice，hear等。
例如：
1.They heard the children sing that morning.
→The children were heard to sing that morning.
2.The teacher made the little boy stand in the classroom.
→The little was made to stand in the classroom by the teacher.
3.We saw them play football just now.
→They were seen to play football just now.
4.We often hear her sing in English.
→She is often heard to sing in English.
一感二听三让四看半帮助
一感:feel
二听:hear，listen to
三让:make，have，let
五看:see，watch，look at，observe，notice
半帮助:help(指其后的不定式可带可不带to)
这些动词后,如跟不定式作宾补,则在主动语态中，不定式不带to,但在被动语态中要带to,如:
He felt something move beside him...
他感觉身边有什么东西在动.
I heard the girl sing an English song.
He often makes his sister cry.
His sister is often made to cry.
Have you seen a young man pass by
Mother watched her son play in the park.
Can you help me (to) carry the box?
Let me have a try.
Please listen to the man tell the story.
