1)表示与现在事实相反的条件和结果，：如：

If I Were you， I should go and see the dentist at once. 假如我是你的话，我会马上去看牙科医生。

If they were no gravity, we should not be able to walk. 假如没有引力，我们就不能行走。

If they had time，they would study Italian too. 假如他们有时间的话，他们也会学意大利语的。

If She knew German，She would read Marx and Engels in the original假如她懂德语，她就会读马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. 假如他知道的话，他会告诉我的，但是他一点也不知道。

2)表示与过去事实相反的条件和结果。如：

If I had known of your arrival I should have met you at the station.如果我早知道你要来，我会去车站接你的。

If you had come a few minutes earlier，you would have met him.要是你早来几分钟的话，你就见到他了。

If She hadn't been so strict with herself, she wouldn't have made such great progress.她要是对自己要求不严格，她就不会有这样大的进步。

If the child had fallen through the ice, he would have drowned. 假如这孩子掉到冰窑里，他定会淹死了。

3)表示与将来事实可能相反的条件和结果。其用法和表示与现在事实相反的条件和结果相同。如：

If it Were Sunday tomorrow，my brother would go skating at the Beihai Park.如果明天是星期天，我弟弟会去北海公园滑冰的。

If you dropped the glass，it would break.你如将杯子掉下来，它就会打碎的。

What Would happen if I put the paper on the fire? －It Would bum.我若把纸放在火上会怎么样?--纸就会烧着。

[注]在表示与将来事实可能相反的条件从句中，亦可用were to + 动词原形(比较正式，常用于书面体中)和should + 动词原形。如：

If you were to come tomorrow，I might have time to see you.你如果明天来的话，我或许会有时间见你。

If I were to see her tomorrow，I would tell her about your decisions. 我明天如见到她，就把你的决定告诉她。

If it should rain tomorrow，what would we do?明天万一下雨，我们怎么办?

If you should meet George，tell him I want to see him.你如见到乔治，告诉他我要见他。