虚拟语气用在一般现在时，
从句IF+S+一般过去时,主句S+SHOULD/WOULD+V
例子：IfIstayedinthatschool,Iwouldpasstheexam.
虚拟语气用在一般过去时，
从句IF+S+过去完成时，主句S+SHOULD/WOULDhavedone
例子：Ifyouhadbeenhereyesterday,youwouldhavemethim.
虚拟语气用在一般将来时，
从句IF+S+was/weregoingtodo,主句S+SHOULD/WOULD+V
例子:ifitwasgoingtorain,wewouldn'tgoonapicnicthisSunday.
