be difficult to do的不定式表被动
类似于 be difficult to do 结构中，若主语与其后的不定式为动宾关系，那么该不定式习惯上要用主动形式表示被动意义。如：
She was difficult to understand at times. 有时候她很不容易理解。
能用于此结构的形容词主要有convenient, dangerous, difficult, easy, hard, impossible, interesting, nice, pleasant, safe, unpleasant 等。这类结构的特点是句子主语必须其后不定式的逻辑宾语。如：
The poem is easy to understand. 这首诗容易理解。
Your writing is impossible to read. 你的书写没法认。
He is hard to get along with. 他这个人很难相处。