To talk about past situations that did not happen, we use a past perfect tense (had + past participle) in the if-clause and would have + past participle in the result clause.

If you had asked him, he would have helped you. (NOT If you would have asked him…) (NOT If you asked him…)
If you had studied harder, you would have passed your exam.
If she hadn’t helped me, I would have been in real trouble.

The same structure can be used to talk about present or future situations which are no longer possible.

If my mother had been alive, she would have turned 90 next year.

In unreal conditional sentences, we can use could and might. Could means ‘would be able to’ and might means ‘would perhaps’ or ‘would possibly’.

If I had another $600, I could buy a car. (= If I had another $600, I would be able to buy a car.)
If you asked me politely, I might get you a drink.

Could have… and might have… can be used in result clauses to talk about past situations that did not happen.

If I had run a bit faster, I could have won.
If he had asked politely, I might have lent him my car.