if的用法：表示条件，意思是如果；表示虚拟，意思是假如；表示是否的含义，和whether相同用法，连接宾语从句。
1.(表示条件)如果,主句用将来时,if从句用现在时表示将来
Hewillcomeifyouinvitehim.
如果你请他,他会来的.
2.(表示虚拟)假如,要是
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.
3.是否=whether连接宾语从句
Iwonderifsheisill.
不知她是否病了.
