hard, difficult, easy, fit, necessary, dangerous, comfortable, heavy这种用在“sth/sb be+这些形容词+to do”这种情况的to do是被动，比如You are so heavy to lift up.举起你来真重（你被举起来）。
to let, to blame, to rent, remain to do等常用主动结构表示被动含义。比如I am to blame.这事儿怪我（我是被怪）。
sb/sth+have/has+sth to do.也是表示被动。比如I have so much work to do.我有那么多活要干（活被干）。
too...to和enough的句子中。比如The coat is too large to wear.这件外套太大了（外套被穿）。The couch is not big enough for me to sit in.沙发对我来说太小了（沙发被坐）。