My parents don’t have their own company so I can’t work for them.
If they had their own company, I would work for them.
David can’t afford to go to Australia. He is badly paid.
If David wasn’t/weren’t badly paid, he would (be able to) / could afford to go to Australia.
I’m not going to ask Zack because he won’t help me.
If I asked Zack, he wouldn’t help me.