1.与现在事实相反的假设

若表示与现在事实相反的假设，则条件状语从句中用过去式，主句中用“would/should/could/might+动词原形”。

When water freezes，its density decreases．If it did not，the oceans would be frozen solid．(人教大纲版高二下第20页)

当水结冰时，它的密度就减小。如果密度不减小，整个海洋就会冻结成固体。

If he were here，we could ask him．

要是他在这儿，我们就能问他。

“If I were you，”Xiaohua says，“I would give an AIDS patient a hug．(人教大纲版高二上第52页)

“如果我是你，”小华说，“我就拥抱艾滋病人一下。”

I am sorry I am busy now．I would certainly go if I had time．

很抱歉我很忙。我要是有时间，我肯定会去的。

If you were working harder，you wouldn't be cold．(人教大纲版高三第57页)

要是工作努力一点，你就不会冷了。

提示：

在从句中be多采用were这个形式，不管是什么人称。

If I were you，I wouldn't lose heart．

我要是你，我是不会灰心的。

If he were here，everything would be all right．

他要是在这儿，一切都没问题了。

What would you do if you were in his place?

你要是处在他的位置你会怎么办?

If it weren't raining，we wouldn't stay indoors．

要是现在不下雨，我们就不会呆在屋子里了。

但在非正式场合，特别是在口语中，第一、第三人称单数也常用was。

If he was here，we might ask him．

如果他在这儿，我们可能会问他的。

If I was in her place，I would think differently．

我要是处在她的地位，我会有不同的想法。

但总是说“if I were you”。