某些“be+形容词+to do”结构中的不定式通常要用主动形式表示被动意义：
The question is difficult to answer. 这问题很难回答.
Your writing is impossible to read. 你的书写没法认.
Do you think the water is safe to drink? 你认为这水喝起来安全吗?
这类结构的特点是句子主语就是其后不定式的逻辑宾语,按理说其中的不定式要用被动形式,但习惯上却要用主动表被动.这类形容词常见的有awkward, convenient, dangerous, difficult, easy, hard, impossible, interesting, nice, pleasant, safe, tough, tricky, unpleasant 等.