经典练习：
1、It’s already six o’clock now. Don’t you think it’s about time____?
A)we are going home
B)we go home
C)we went home
D)we can go home
2、If only the committee____the regulations and put them into effect as soon as possible.
A)approve
B)will approve
C)can approve
D)would approve
3、It’s necessary____the dictionary immediately.
A)that he will return
B)that he returned
C)that he return
D)that he has to return
4、She must have had an accident, or he____then.
A)would have been here
B)had to be here
C)should be here
D)would be here
5、I apologize if I ____you, but I assure you it was unintentional.
A)offended
B)had offended
C)should have offended
D)might have offended

解析：
1、答案选C)。在it’s about time that…..句型中，定语从句的谓语动词应用过去时来表示虚拟语气。所以C项是正确的。
2、答案选D)。if only表示一种愿望，谓语用“would+动词原形”表示对将来事实的愿望，所以D是正确的。
3、答案选C)。在由it is necessary 等引导的主语从句要使用虚拟语气，虚拟语气的谓语动词用“should+动词原形”或动词原形。
4、答案选A)。由or引出的是与事实相反的假设条件，因此空缺处应填入表示虚拟语气的谓语动词。前一个分句中表示的是过去的时间，所以后一分句要用would have done的形式。所以A是正确的。
5、答案选B)。本题中but分句中的it was 表明事情发生在过去，因此条件句应用过去完成时，表示与过去事实相反，所以B是正确的。