1)表示與現(xiàn)在事實(shí)相反的條件和結(jié)果，：如：
If I Were you， I should go and see the dentist at once. 假如我是你的話，我會馬上去看牙科醫(yī)生。
If they were no gravity, we should not be able to walk. 假如沒有引力，我們就不能行走。
If they had time，they would study Italian too. 假如他們有時間的話，他們也會學(xué)意大利語的。
If She knew German，She would read Marx and Engels in the original假如她懂德語，她就會讀馬克思和恩格斯的原著了。
He would tell me if he knew， but he does not，know anything about it. 假如他知道的話，他會告訴我的，但是他一點(diǎn)也不知道。
2)表示與過去事實(shí)相反的條件和結(jié)果。如：
If I had known of your arrival I should have met you at the station.如果我早知道你要來，我會去車站接你的。
If you had come a few minutes earlier，you would have met him.要是你早來幾分鐘的話，你就見到他了。
If She hadn't been so strict with herself, she wouldn't have made such great progress.她要是對自己要求不嚴(yán)格，她就不會有這樣大的進(jìn)步。
If the child had fallen through the ice, he would have drowned. 假如這孩子掉到冰窯里，他定會淹死了。
3)表示與將來事實(shí)可能相反的條件和結(jié)果。其用法和表示與現(xiàn)在事實(shí)相反的條件和結(jié)果相同。如：
If it Were Sunday tomorrow，my brother would go skating at the Beihai Park.如果明天是星期天，我弟弟會去北海公園滑冰的。
If you dropped the glass，it would break.你如將杯子掉下來，它就會打碎的。
What Would happen if I put the paper on the fire? －It Would bum.我若把紙放在火上會怎么樣?--紙就會燒著。
[注]在表示與將來事實(shí)可能相反的條件從句中，亦可用were to + 動詞原形(比較正式，常用于書面體中)和should + 動詞原形。如：
If you were to come tomorrow，I might have time to see you.你如果明天來的話，我或許會有時間見你。
If I were to see her tomorrow，I would tell her about your decisions. 我明天如見到她，就把你的決定告訴她。
If it should rain tomorrow，what would we do?明天萬一下雨，我們怎么辦?
If you should meet George，tell him I want to see him.你如見到喬治，告訴他我要見他。