4种情况,1,当主语和谓语动词在逻辑上存在主谓关系的时候,用主动表示被动,如,I have something to send to my mother.这 里的something 和send 在逻辑上存在这样的关系.要不就应该是i被动了.2,it + 形容词+to ,结构中,如,it is hard to learn English. 3, 谓语动词+ 疑问代词 用法中,如,I don't know how to read this book? 4, to let ,to blame,结构中. 当然你说的情况：如need , want require ,demand, 等,后面接动词的ing 形式表示被动,如need wshing = something need to be washed. 都是手打的.希望对你有用,希望你能采纳.
