we all thought ___that the football match should have been put off.

a
it形式宾语,代表真正的宾语（后面的that从句）,a pity宾语补足语
---宾语太长,为to do.,或者为宾语从句时,后面又有形容词或名词作宾语补语时,用it作形式宾语,跟宾语补足语,然后是真正的宾语
句型为think sb/sth (to be) sth,think this (to be) a pity我们都把...看作一件憾事.
what a pity （it is that ...)是感叹句,不能保留感叹词what直接改成间接引语.
---He said,“What a pity it is that ...!"
不能改成：he thought what a pity it was...
可以改为：He exclaimed that it was a pity that...
补充：should have been put off不是虚拟语气,是用should不是某种情绪,如,失望