(1)在主动语态中,make ,let ,hear ,see ,watch ,notice 等词,其后跟省略to的动词不定式(动词原形),在变为被动语态时,必须还原动词不定式符号to.如:
We heard him sing this song in the room.
He was heard to sing this song in the room.
The boss made him work 14 hours a day.
He was made to work 14 hours a day.
I see him walk to school
He is seen to walk to school.
（2）带双宾语的动词give,send ,teach ,lend ,show ,pass ,tell等,在变成被动语态时,可以有两种情况.如果用直接宾语作为被动语态的主语,这时,间接宾语前要加介词to;如果用间接宾语做主语,则不加to.如:
A pen was given to me by my friend yesterday
My friend gave me a pen yesterday
I was given a pen by my friend yesterday.
The children are often told some stories by him.
He often tells the children some stories.
Some stories are often told to the children by him.
在完成时态中,如果也符合上面两条的话,也是要用to的.