[Scenery]Sue has lost her watch. She think it may be at Ann’s house.
SUE： I think I left my watch at your house. Have you seen it?
(1) ANN： no， but I’ll have a look when I get home. If I find it， I’ll tell you.
If I find…。， I’ll……
(2) Ann says： If I found a wallet in the street， I’d take it to the police.
If I found……， I’d(=I would)……。.(而不是if I find， I’ll…。.)
这里两个句子有着不同的意思，前者表示Ann觉得她有找到手表的真实可能性。因此才说if I find…。.， I’ll…。.
而后者是完全不同的情况。在这里Ann没有考虑真实的可能性;她在虚拟这一个情景，并不认为真的可以在街上拣到手表。于是才说：if I found……，I’d……
if I do………。和if I did………的区别
用if+过去式(if I found / if you were / if we didn’t等) 表示虚拟的事情，而不是指过去时间发生的事情：
What would you do if you won a million pounds?
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.
I’d be very frightened if somebody pointed a gun at me.(而非if somebody would point)
If I didn’t go to their party， they’d be offended(而非if I wouldn’t go)
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.”
Would(‘d) / wouldn’t用于句子的主句(不是if从句)
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.