当主句与if条件句中的动作都发生在过去时,主句谓语动词就要使用haddone形式,if条件句中谓语动词则要使用wouldhavedone形式.e.g.IfIhadworkedhard,Iwouldhavepassedalltheexams.
当主句以及if条件句中的动作都发生在现在时,主句的谓语动词必须使用did(be动词用were),if条件句动词须使用woulddo形式.e.g.
Ifhecameontimeforthemeeting,Iwouldtreatyoutoarichmeal.
IfIwereyou,Iwouldgowithhim.
当主句和if条件句的动作都发生在将来时,主句动词可以使用did,shoulddo或weretodo形式,if条件句中动词须使用woulddo形式.
e.g.Ifyoukept/should/keepweretokeepyourpromise,youwouldearnmorethantrust.