it的主语和宾语是什么

it用作形式主语和宾语是因为英语句子通常要求美观，不喜欢头大，肚子大的句子，所以通常把很长的主语和宾语放到句子的后面，本来应该是主语，宾语的地方用it来代替. 如: It is difficult for me to do the homework alone. to do the homework alone 是真正的主语 I think it impossible for me to do the homework alone. 同样这里的to do the homework alone 是真正的宾语. 理解了吗?^_^