1.此句型是一个简单句,句中主语同时还做不定式的宾语,即that box就是carry的宾语,所以carry后面不能再出现it.
此固定句型可以归纳为:sb/sth be + 形容词 + to + 动词原形(+ 介词).
再如:English is difficult to learn.(English还做learn的宾语)
The man is easy to work with.(The man还做(work) with的宾语)
2.如果是含有状语从句的主从复合句,则it不能省略,如:
如:That box is so heavy that I can't carry it.
再如:This is such a heavy box that I can't carry it.
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