現(xiàn)在① 表示與現(xiàn)在事實相反的情況：
主語+should/would/could/might+do+其他+if+主語+did/be(were)+其他
例：
1.If I were you,I would take an umbrella.如果我是你,我會帶把傘.（事實：我不可能是你）
2.If I knew his telephone number,I would tell you.如果我知道他的電話號碼,我就會告訴你.（事實：不知道）
3.If there were no air or water,there would be no living things on the earth.如果沒有水和空氣,地球上就不會有生物.(事實：地球上既有空氣也有水)
過去②表示與過去事實相反的情況
主語+should/would/could/might+have done+其他+if+主語+had done +其他
例：
1.If I had got there earlier,I should/could have met her.如果我早到那兒,我就會見到她.（事實：去晚了）
2.If he had taken my advice,he would not have made such a mistake.如果他聽我的勸告的話,就不會犯這樣的錯誤了.（事實：沒有聽我的話）
將來③表示對將來情況的主觀推測（可能相反或可能性很小）
主語+should/would/could/might+do+其他+if+主語+did/should/were to do+其他
例：
1.If he would come here tomorrow,I should/would talk to him.如果他哪天來這兒的話,我就跟他談談.（事實：來的可能性很?。?br/>2.If there were a heavy snow next Sunday,we would not go skating.如果下周日下大雪,我們就不能去滑冰了.（事實：不知能否下雪）
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總結(jié)：
現(xiàn)在① 表示與現(xiàn)在事實相反的情況：
If it did ..,I should do..
過去②表示與過去事實相反的情況
If it had done ..,I+should have done..
將來③表示對將來情況的主觀推測（可能相反或可能性很?。?br/>If it would do..I should/would do..