He wasn’t able understand many of the formulas in chemistry until he started to revise seriously.
HE WASN'T ABLE TO UNDERSTAND...
He shouldn’t have ate all those sweets.
SHOULDN'T HAVE EATEN...
He’s been under a lot of stress at work, so he hasn’t can go out much recently.
HE HASN'T BEEN ABLE TO GO...
Did you manage run the marathon in less than three hours?
DID YOU MANAGE TO RUN...
He didn’t has to buy a ticket because he’s a student.
HE DIDN'T HAVE TO...
