1）时态：可以表示过去，现在和将来的情况。它的基本特点是时态退后。
a. 同现在事实相反的假设。
句型：条件从句 主句
一般过去时 should(would)+动词原形
Iftheywerehere,theywouldhelpyou.b. 表示于过去事实相反的假设。
句型：条件从句 主句
过去完成时 should(would)have+过去分词
Ifshehadworkedharder,shewouldhavesucceeded.
Thericewouldnothavebeenburntifyouhadbeenmorecareful.
IfmylawyerhadbeenherelastSaturday,hewouldhavepreventedmefromgoing. Ifhehadcomeyesterday,Ishould/wouldhavetoldhimaboutit.
含义：Hedidnotcomeyesterday,soIdidnottellhimaboutit.
Ifhehadnotbeenillandmissedmanyclasses,hewouldhavemadegreaterprogress.
含义：Hewasillandmissedmanylessons,sohedidnotmakegreaterprogress. c. 表示对将来的假想
句型： 条件从句 主句
一般过去时 should+动词原形
were+不定式 would+动词原形
should+动词原形
Ifyousucceeded,everythingwouldbeallright.
Ifyoushouldsucceed,everythingwouldbeallright.
Ifyouweretosucceed,everythingwouldbeallright.
