使用if 的句子一定都是虚拟语气。而if 和then 的结构有多种形式:

  ◆ IF you study diligently, [THEN] you will score highly.

  ◆ You will score highly IF you study diligently.

 

Then 在虚拟语气中常常被省略,同是if 也不见得都在首句中出现。下面是常见的if then 的五种结构(在这五种结构中不能在if 引导的句子中出现would ,而且一整句话都不能出现should 在if then 结构中):

  1. 不存在不确定性的情况:

  ◆ IF Sophie EATS pizza, THEN she BECOMES ill.

  ◆ IF Present, THEN Present.

 

  这种情况比较像whenever :WHENEVER Sophie EATS pizza, she BECOMES ill.

  2. 存在一些不确定性的情况:

  ◆ IF Sophie EATS pizza, THEN she MAY BECOME ill.

  ◆ IF Present, THEN Can or May.

 

  3. 将来时不存在不确定性

  ◆ IF Sophie EATS pizza tomorrow, THEN she WILL BECOME ill.

  ◆ If Sophie HAS EATEN pizza, then she WILL BECOME ill.

  ◆ IF Present, THEN Future.

 

  4. 将来时不太可能发生

  ◆ IF Sophie ATE pizza tomorrow, THEN she WOULD BECOME ill.

  ◆ IF Hypothetical Subjunctive, THEN Conditional.

  这里作者认为Sophie 不大可能吃pizza,这里用到了would,这里的would 可以用could 来替代。

 

  5. 从不可能发生

  ◆ IF Sophie HAD EATEN pizza yesterday, THEN she WOULD HAVE BECOME ill.

  ◆ IF Past Perfect, THEN Conditional Perfect.

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