混合條件句

 主句與從句的動作發生在不同的時間，這時主，從句謂語動詞的虛擬語氣形式因時間不同而不同，這叫做混合條件句。
if you had asked him yesterday, you would know what to do now.
　 （從句與過去事實相反，主句與現在事實相反。）
if it had rained last night (過去), it would be very cold today (現在).