1 If they wouldn't be so tired, they would work harder
2 If they knew the answer, they would've told us
3 If he hadn't lost his keys, he could've opened the door
4 if I'm not good at maths I won't become an engineer
5 If he wouldn't be working so hard, he would have time to spend with his family
6 If they had learnes for today, they wouldn't have gotten bad grades