4. if條件句中的虛擬語氣形式
1）if 非真實條件句所表示的假設(shè)則是不可能或不大可能發(fā)生或?qū)崿F(xiàn)的，句中的主句與從句都用虛擬語氣。
在if非真實虛擬條件句中，主句和從句謂語動詞主要有下面幾種形式：
假設(shè)類型 條件從句謂語動詞形式 主句謂語動詞形式
與現(xiàn)在事實相反 動詞過去時（be 用 were） should（would，could，might） + 動詞原形
與過去事實相反 had + 過去分詞 should（would，could, might）+ have + 過去分詞
與將來事實可能相反 動詞過去時（should+動詞原形，were to + 動詞原形） should（would，could，might）+ 動詞原形
if there were no gravity, we should not be able to walk.
假如沒有引力，我們就不可能行走。
i wouldn’t have known what these were for if i hadn’t been told.
假如別人不告訴我，我就不知道這些東西是干什么的了。
2） 正式文體中，有時可把虛擬條件句中的連詞if省去，而將were，had, should等助動詞（不包括行為動詞）提到主語前面。如果句中沒有were，had或should時，既不能省略if，也不能倒裝。例如：
were i to meet him tomorrow (= if i were to met him tomorrow), i should ask him about it.
要是我明天見到他，我就會問他這件事的。
had i had the money last year (= if i had had the money last year), i would have bought the house.
如果我去年有了這筆錢，我就買那所房子了。
should there be any trouble with the boiler, the automatic controlling unit would cut off the fuel oil supply.
假如鍋爐出問題的話，自控裝置會自動切斷燃油的供給。
3） 通常情況下，在非真實條件句中主句和從句的謂語動詞所指時間是一致的，但有時也可能指不同的時間，這時要根據(jù)上下文的意思采用不同的謂語動詞形式。例如：
if i were you, i would have taken his advice.
我要是你，我就采納了他的建議。（從句指現(xiàn)在，主句指過去）
if the weather had been more favorable, the crop would be growing still better.
如果氣候更適宜一些，莊稼會長得更好。（從句指過去，主句指現(xiàn)在）