注意：当 need, want, require, worth后面接doing也可以表示被动。
Your hair wants cutting. 你的头发该理了。
The floor requires washing. 地板需要冲洗。
The book is worth reading. 这本书值得一读。
典型例题
The library needs___, but itll have to wait until Sunday.
A. cleaning B. be cleaned C. clean D. being cleaned
答案A. need +n /to do，need + do，当为被动语态时，还可need + doing. 本题考最后一种用法，选A。如有to be clean 则也为正确答案。
典：done，不可能已经。must not do 不可以(用于一般现在时)。