1. they don't behave themselves so their parents are not happy about that

-> if they behaved themselves, their parents would be happy about that.

2. she didn't say sorry so he was angry

-> if she had said sorry, he wouldn't have been angry.

3. i am not a rich businessman and t can't afford to buy an expensive car

-> if I was a rich businessman, I could ( afford to ) buy an expensive car.

4. it rained last night so i didn't go to the barbecue

-> if it hadn't rained last night, I would have gone to the barbecue.

5. my parents do not allow me to go, so i have to stay at home

-> if my parents allowed me to go, I wouldn't ( have to ) stay at home.

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