The poor couple know____is no good complaining about their family background
问题描述：
The poor couple know____is no good complaining about their family background
The poor couple know is____no good complaining about their family background
为什么用it啊
答
it在这里指代complaining about their family background 作后面从句的形式主语为什么不能用what 作主语啊it is no good+doing固定句型