Compare these examples:

1.) Sue has lost her watch. She thinks it may be at Ann's house.
SUE: I think I left my watch at your house. Have you seen it?
ANN: No, but I'll have a 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....

2.) Ann says: If I found a wallet on the street, I'd take it to the police.

This is a different type of situation. Here, Ann is not thinking about a real possibility; she is imagining the situation and doesn't expect to find a wallet in the street.
So she says: If I found ..., I'd (= I would) ... (not If I find ...,I'll)

A. When you imagine something like this, you use IF + past (if I found/if you were/if we didn't etc.). But the meaning is not past:

* What would you do if you won a million pounds? (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 offended if I didn't go.
* Sarah has decided not to apply for the job. She isn't really qualified for it, so she probably wouldn't get it if she applied.

B. We do not normally use WOULD in the if part of the sentence:
* 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 offended. (not 'If I wouldn't go')

But it is possible to say " IF... WOULD " when you ask somebody to do something:
* (from a formal letter) I would be grateful if you would send me your brochure as soon as possible.
* 'Shall I close the door?' 'Yes, please, if you would.'

C. On the other part of the sentence (not the if part) we use WOULD ('d)/WOULDN'T:
* If you took more exercise, you'd (= you would) probably feel healthier,
* Would you mind if I used your phone?
* I'm not tired enough to go to bed yet. I wouldn't sleep (if I went to bed now).

COULD and MIGHT are also possible:
* If you took more exercise, you might feel healthier. (= it is possible that you would feel healthier)
* If it stopped raining, we could go out. (= we would be able to go out)

D. Do not use WHEN in sentences like those on this page:
* They would be offended if we didn't accept their invitation. (not 'when we didn't')
* What would you do if you were bitten by a snake? (not 'when you were bitten')

Substitution Drill

Supply the missing words appropriately.

* What you do you won a million pounds?
* I don't really want to go to their party, but I probably will go. They be offended I didn't go.
* Sarah has decided not to apply for the job. She isn't really qualified for it, so she probably get it she applied.
* you mind I used your phone?
* What you do you were bitten by a snake?
* I be very frightened somebody pointed a gun at me.