四．句型结构
1. They are easy to learn.
形容词cheap, dangerous, difficult, easy, expensive, interesting, nice, pleasant, strange, fit等后常常跟不定式的主动形式，表被动意思；不及物动词要加上相应的介词。
Mary is pleasant to talk to.
The river is dangerous to bathe in.
The question to answer.
Dirty isn’t fit to drink.
