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.
