一、與現在事實相反

若與現在事實相反，虛擬語氣的條件從句的謂語用過去式(be通常用were)，主句謂語用“should (would, could, might)+動詞原形”。如：

If you took a taxi，you"d get there quicker. 如果你坐計程車去，你可以快一點到那裡。(但你不坐)

If I knew her number I could ring her up. 要是我知道她的電話號碼，我就可以給她打電話了。(可惜我不知道)

二、與過去事實相反

若與過去事實相反，虛擬語氣的條件從句的謂語用過去完成時(had+過去分詞)，主句謂語用“should (would, could, might)+have+過去分詞”。如：

If I"d left sooner，I"d have been on time. 要是我早點動身，我就準時到了。(但我動身太遲了)

If we had found him earlier we could have saved his life. 要是我們當時早點找到他的話，我們就可以救活他。(可惜我們找到他太晚了)

三、與將來事實相反

若與將來事實相反，條件從句的謂語用過去式(be通常用were)，主句謂語用“should (would, could, might)+動詞原形”。