一、虛擬語氣用于表語從句及同位語從句中
當advise, decision, desire, demand, idea, order, motion, necessity, insistence, instrution, plan, preference, proposal, pray, recommendation, requirement, resolution, suggestion, understanding等名詞后接接表語從句或同位語從句時，從句的謂語用“should+動詞原形”的形式來構(gòu)成虛擬語氣。
例：The professor gave order that the test be finished before 7:30.
教授指示考試須在七點半以前結(jié)束。
例：The suggestion that the mayor present the prizes was accepted by everyone.
二、虛擬語氣用于方式狀語從句中
在as if, as though 引導的方式狀語從句中，虛擬語氣的形式是：用過去時表示與現(xiàn)在事實相反或或?qū)ΜF(xiàn)在情況有所懷疑；用過去完成時表示對過去事實的假設(shè)。
例：It seems as if it were spring.
現(xiàn)在好像是春天似的。
例：She appeared as if she had known nothing about it.
她看上去對此一無所知。
三、虛擬語氣用于目的狀語從句中
在lest, for fear that, in case 等引導的目的狀語從句中，謂語動詞用“should+動詞原形”來構(gòu)成虛擬語氣；在so that, in order that 等引導的目的狀語從句中，謂語動詞除了用“should+動詞原形”外，還可以用“might, could+動詞原形”來構(gòu)成虛擬語氣。
例：She’s studying hard for fear that he should fall behind.
她努力學習，生怕跟不上。
四、虛擬語氣用于讓步狀語從句中
在讓步狀語從句中，謂語動詞多用動詞原形來構(gòu)成虛擬語氣。
注：由even if 或even though引導的讓步狀語從句中，其從句的謂語動詞形式與含有非真實條件從句的主從句中的謂語動詞一樣來構(gòu)成虛擬語氣。

經(jīng)典練習：
1、Our idea is that the match____.
A)would put off
B)ought to put off
C)be put off
D)could put off
2、One of the requirements for a fire is that the material_____to its burning temperature.
A)is beated
B)will be heated
C)be heated
D)would be heated
3、That tree looked as if it _____for a long time.
A)hasn’t watered
B)didn’t water
C)hadn’t been watered
D)wasn’t watered
4、The business of each day, _____selling goods or shipping them, went quite smoothly.
A)it being
B)be it
C)was it
D)it was

參考答案:

1、答案選C)。根據(jù)虛擬語氣的語法要求,that 從句中的動詞用動詞原形或“should+動詞原形”的形式。所以C為正確選項。
2、答案選C)。requirement 后接一表語從句，從句的謂語用“should+動詞原形”的形式來構(gòu)成虛擬語氣，四個選項中只有C符合。句意：產(chǎn)生火的必要條件之一是物質(zhì)被加熱到它的燃點。
3、答案選C)。as if引導一個表示與過去事實相反的虛擬語氣的句子。主句的謂語動詞是一般過去時，所以as if 后虛擬語氣的謂語動詞要用過去完成時，所以C是正確的。
4、答案選B)。be it selling goods or shipping them 是表示讓步意義的虛擬倒裝句，它是由whether it (might)be selling goods or shipping them這個句子省略whether后再將be 移至主語之前得來的。其它幾項均不符合句子結(jié)構(gòu)。句意：無論是售貨還是運貨，每天的業(yè)務(wù)都很不錯。