Compare these examples:
Sue has lost her watch. She tells Ann:
Sue : I think I felt my watch at your house. Have you seen it?
Ann : No, but I’ll look when I get home. If I find it, I’ll tell you.
In this example, Ann feels there is a real possibility that she will find the watch. So she says:
If I find …, I’ll
If I found a wallet in the street, I’d take it to the police station.
This is a different type of situation. Here, Carol doesn’t expect to find a wallet in the street.
She is imagining a situation that will probably not happen. So she says:
If I found …, I’d (= I would) … (not if I find …, I’ll)
When you imagine something like this, you use if + past (if I found / if there was / if we didn’t, etc.).
But the meaning is not past:
What would you do if you won a million dollars?
(we don’t really expect this to happen)
I don’t really want to go to their party, but I probably will go. They’d be hurt if I didn’t go.
If there was (or were) an election tomorrow.
who would you vote for?
We do not normally use would in the if part of the sentences:
I’d be very frightened if somebody pointed a gun at me. (not if somebody would point)
If I didn’t go to their party, they’d be hurt. (not If I wouldn’t go)
In the other part of the sentence (not the if part) we use would (‘d) / wouldn’t:
If you got more exercise, you’d feel better,
I’m not tired. If I went to bed now, I wouldn’t sleep.
Would you mind if I used your phone?
Could and might are also possible:
If you got more exercise, you might feel better. (= it is possible that you would feel better)
If it stopped raining, we could go out. (= we would be able to go out)
Do not use when in sentences like the ones on this page:
They’d be hurt if I didn’t go to their party. (not when I didn’t go)
What would you do if you were bitten by a snake? (not when you were bitten)