we think it our duty to help others

问题描述:

we think it our duty to help others
为什么用 it 不用 this that

this 和that 不能做宾语,但可做主语.
而it既可以做主语又可以作宾语.
如果用this 或that的话,必须是:we think this/that is our duty to help other.