例：
If you had taken your medicine yesterday, you would be quite all right now.
如果你昨天吃了药的话，现在就已经痊愈了。(从句表示与过去事实相反的假设，主句则表示与现在事实相反。)
经典练习：
1、If I hadn’t stood under the ladder to catch you when you fell, you____now.
A) couldn’t have smiled
B) wouldn’t be smiling
C) didn’t smile
D) won’t smile
2、____for the timely investment from the general public, our company would not be so thriving as it is.
A) Had it not been
B) Were it not
C) Be it not
D) Should it not be
3、If he knew this, it____be by accident.
A) would have to
B) didn’t to
C) won’t have to
D) woud have had to

解析
1、选B。从主句的时间状语now以及从句的谓语可知，本题为一错综时间虚拟语气，从句表示与过去事实相反的假设，主句则表示与现在情况相反，故B项为正确答案。
2、选A。根据本题句意逻辑，条件从句的行为发生在主句之前，须用过去时的虚拟语气形式，故A项为正确答案。同时，它是由把if省略，助动词提前而形成倒装。B、C、D三项均不合题意。本题句意是：要不是能及时从社会获得投资，我们公司也不会像现在这样繁荣了。
3、选D。由从句的谓语可知，本题为一错综时间虚拟语气，从句表示与现在事实相反的假设，主句则表示与过去事实相反的设想。故D项为正确答案。本句题意：假如他已知此事，那一定是偶然的。