First read the rules!
1) You regret (or are sorry about a present situation)
EXAMPLE: I wish I was slim!
FORM: I wish + past simple
2) You regret a past action or situation
EXAMPLE: I wish I hadn’t bought that jacket. / I wish I had saved my money.
FORM: I wish + past perfect
3) You want another person or thing to do – or not to do – something. This is often when you are annoyed.
EXAMPLE: I wish he wouldn’t play loud music at night. / I wish it would stop raining!
FORM: I wish + would + infinitive