經典練習：
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、答案選C)。在it’s about time that…..句型中，定語從句的謂語動詞應用過去時來表示虛擬語氣。所以C項是正確的。
2、答案選D)。if only表示一種愿望，謂語用“would+動詞原形”表示對將來事實的愿望，所以D是正確的。
3、答案選C)。在由it is necessary 等引導的主語從句要使用虛擬語氣，虛擬語氣的謂語動詞用“should+動詞原形”或動詞原形。
4、答案選A)。由or引出的是與事實相反的假設條件，因此空缺處應填入表示虛擬語氣的謂語動詞。前一個分句中表示的是過去的時間，所以后一分句要用would have done的形式。所以A是正確的。
5、答案選B)。本題中but分句中的it was 表明事情發生在過去，因此條件句應用過去完成時，表示與過去事實相反，所以B是正確的。