MODALS & PERFECT MODALS
I'm sure he's at home --- HE .................... AT HOME
HE MUST BE AT HOME
It's impossible that he knew the problem ---- HE ............. THE PROBLEM
HE CAN'T HAVE KNOWN THE PROBLEM
We have bought more food than was necessary for the party ---- WE _______ FOR THE PARTY.
WE NEEDN'T HAVE BOUGHT MORE FOOD FOR THE PARTY.