If Sentence

a. Compare This Examples:

Tom : I think i left my lighter at your house. Have you seen it ?
Ann : No, but I'll look. If I find it, I'll give it to you.

In this example there is a real possibility that Ann will find the lighter. So she says "If I find...I'll..."

Ann : If I found a $100 bill on the street, I would keep it.

This is a defferent type of situation.  Ann is not thinking about a real possibility; She is imagining the situation. So She says "If I found...I  would...."( Not "If I find.....I'll....").

When you imagine a future happening like this, you use a past tense form (did/came/found,etc) after if. But the meaning is not past :

• What would you do if you won a million dollars ?
• If we didn't go to their party next week, they would be very angry.
• Ann wouldn't lend me any money I asked her.

b. We don't normally use would in the if part of sentence :

• If we didn't go to their party next week, they would be angry. (not if we wouldn't go) 

Sometimes it is possible to say if...would, especially whwn you ask someone to do something in a formal way :

• I would very grateful if you would send me your brochure and price list as soon as possible. (form a formal letter)

c. In the other part of the sentence (not the if part) we use would/wouldn't. would is often shortened to 'd, especially in spoken English.

• If you stopped smoking, you'd probably feel healthier.
• They wouldn't come to the party if you invited them.

You can also use could or might :

• They might be angry if I didn;t visite them. (=perhaps they would be)
• If it stopped raining, we could go out.(=we would be able to go out)

d. Don't use when in the sentence like the one in the unit:

• Tom would be angry if I didn;t visite him. (not when i didn't visite )
• What would you do if you were bitten by a snake ? (not when you were)