后跟动词不定式做宾补省to 的动词，在变为被动句时 必须加上to. 动词有以下几个：
四看：see, watch, notice, look at
三让：let, make, have
二听： hear, listen to
into the room.
She was seen into the library.
Practice: fill in the blanks.
1. He often made us ____ (laugh).
2. I had my bike _____ (repair) yesterday.
3. The boss made the workers ________ (work) ten hours very day.
4. They are heard _______ (sing) in the next room.
5. Let me _____ (help) you.
6. I watch him _____ (play) the piano now.
7. My mother doesn’t have me ____ (do) any housework.
8. We usually help the old ______ (clean) their rooms.