这类系表结构后不定式要用主动式表被动意义
先看一道题：
This book is difficult __________.
A. to understand B. to be understand C. to understand it D. being understood
此题应选A。容易误选B。从意思上看，本句主语与其后不定式有动宾关系，所以不少考生便误选了不定式的被动式，但是这里有一点特殊之处。
按照英语习惯，由easy，hard，nice，difficult，amusing，interesting，dangerous，impossible，important，pleasant，等形容词构成的系表结构后，接用不定式时，通常要用主动式表示被动含义：
English is not easy to learn. 英语不容易学。
This question is difficult [hard] to answer. 这个问题很难回答。
That bird is impossible to catch. 那只鸟不可能被抓住。
The river is dangerous to swim in. 在这条河里游泳很危险。
Football is interesting to watch. 看踢足球很有意思。
以上结构可以换成用 it 作形式主语的句型：
正：English words are difficult to remember.
正：It is difficult to remember English words.
写那样一本书很容易。
正：Such a book is easy to write.
正：It is easy to write such a book.(www.hxen.com)