这是个主语从句.划线的地方填 that .it 在句中作形式主语,而that 作为真正的主语后置.出现这种情况,是因为真正的主语过长,为了避免头重脚轻的现象,所以用it 做了形式主语.
it worried the mother a lot that she couldn't afford the medical treatment for her son.
可以= That she couldn't afford the medical treatment for her son worried the mother a lot.