虛擬語氣的句型分類和用法
虛擬語氣可以粗略地分為兩大類：
第一類：should 型虛擬式 第二類：were 型虛擬式
1.should 型虛擬式：should + 原形動詞(往往省去should, 特別是在考試時)
這類虛擬式最多用在以下詞或句型中：
(1) It is necessary that…的主語從句中，如：
It is necessary that the chief should be present personally at the meeting.(主任有必要親自參加這次會議。)
只要看到"necessary"，馬上要想到這是"should型"虛擬式。
記住下面構成"should型"虛擬式的詞：
essential (必要的) / important(重要的) / necessary(必要的) / imperative(迫切的)/ impossible(不可能的)/ natural(自然的)/ preferable(更好的)/ advisable(合理的)/ desirable(理想的)/ urgent(緊迫的)/ incredible(難以想象的)
(2) order (命令) / recommend(推薦)/ suggest(建議)/ demand(要求) / command(命令)/ propose(建議)/ require(要求)/ request(請求) / insist(堅持認為)/ urge(主張) / advise(勸告)/ ask(要求)/ prefer(寧愿)/ desire(愿望)/ move(提議)/ vote(提議)
由這些詞構成的賓語從句，主語從句、表語從句、同位語從句或其他有關句型時，都要使用"should型"虛擬式，例如醫(yī)學.全在.線.提供：
It was ordered that the medicines (should) be sent there by plane at once. (主語從句)
The order was that the medicines (should) be sent there by plane at once.(表語從句)
The chief ordered that the medicines (should) be sent there by plane at once.(賓語從句)
The order that the medicines (should) be sent there by plane at once was sent to the branch office.(同位語從句)
(3) 在 in case / lest = for fear that(以免… / 以防…)結構中，例如：
He took a raincoat with him in case it should rain (或用真實語氣：…in case it rained)
She put a wool blanket over the child for fear that (lest) he should catch a cold. (她把一條毛毯蓋在孩子身上，以防他感冒。)
可見，此類虛擬語氣比較容易接受，考試時只要認準了上面的那些字一般就不難作出準確選擇。
就解題而言，考生要注意：1)A的位置往往安排干擾項，如使用must, may, might, could等類詞進行干擾;2)句中往往省去should, 直接用否定詞not + 原形動詞的形式，以"迷惑"考生，例如：
The housemaster(男舍監(jiān))was very strict.He asked that we ______ television on week nights.
A. must not watch
B. not watch
C. should not have watched
D. not be watching
該題中的"ask"意為"要求"，故是個"should型"虛擬式, 正確答案為B.
(4)在it is surprising, disappointing, a pity, a shame, no wonder, simply a miracle + 從句的結構中，表示驚訝、高興或遺憾，例如：
It is surprising that she should look so miserable. (真是奇怪，她竟看上去如此不高興。)
It is a great pity that you should miss such a good film. (你要是錯過了這樣好的電影那真是遺憾。)
