If“是否”等于Whetherif和whether作“是否”之义，用来引导宾语从句时，一般可以互换。但是，在下列情形不能换用：引导宾语从句当宾语从句在主句前面时。如：Whetherhediditornot,Icannottell.宾语从句前置时只能用whether。如：Whetherheliveshere,Iwanttoknow.在decide,discuss等动词后，通常指更whether引导的宾语从句。如：Theydiscussedwhethertheyshouldclosetheshopatthemeeting.Theoldladyhasnotdecidedwhethershewilllivewithherdaughterinthecountryside.出现在介词后面的宾语从句只能用whether来引导。如：ItalldependsonwhetheryoulikesurftheInternet.Theteacherworriedaboutwhetherthegirlcouldpasstheexamthistime.句中含有引导条件状语从句的if时，用whether而不用if。如：Billaskedwhether,ifIgotthejob,I’dmovetoNewYork.若说话人期望得到或估计会得到肯定答复时常用if，而在对答复的肯定与否没有把握或不在意时用whether。如：Heaskedmeifhecouldcome.他问我他可不可以来。Heaskedmewhetherhecouldcome.他问我他是否该来。（暗示他对来不来都无所谓。）7．在有些句子中，用if引导宾语从句可能会产生歧义。试比较：Youshouldtellyourmotherwhetheryouwanttogowithherforaholiday.你应该告诉你母亲你是否想和她一起度假。Youshouldtellyourmotherifyouwanttogowithherforaholiday.如果你想和你母亲一起度假，你就应该告诉她。引导主语从句引导主语从句时，常用whether。如：Whetheritwilldousgoodorharmremainstobeseen.Whethertheywillwinthegameisallthesametome.但是，当采用形式主语it，而将主语从句后置时，用whether和if均可。如：It’snotcleartomewhether/ifshelikesthepresent.(《英语语法大全》P1456)Itwasnotknownwhether/ifhewouldask.(《大学英语语法》P397)引导表语从句引导表语从句时，一般用whether。如：Thequestioniswhethertheycancooperatewithus.Theproblemiswhetheryoucanfinishthejobintime.引导同谓语从句引导同谓语从句时，一般用whether。如：Thequestionwhetherheshouldcomehimselforsendasubstitutemustbedecidedrightnow.Wewillsoondiscusstheproblemwhetherweshouldtakesomeeffectivemeasurestoprotecttheendangeredanimalsandplantsinthisregion.引导让步状语从句引导让步状语从句时，一般用whether。如：Themoonisamoonstill,whetheritshinesornot.Whetherwalkingorsleeping,hebreathesnoisily.Itisdecided,whetherforbetterorworse.与不定式连用在不定式之前用whether.如：Hedoesn’tknowwhethertostayornot.IthasnotbeendecidedyetwhethertogivetheEnglishoraltestthisFriday.与or,ornot连用Whether侧重“选择”，“怀疑”，与or,ornot连用，构成whether…or,whether…ornot等结构。如：Idon’tknowwhetherIshouldgoorstayhere.Whethertheydoitornotmatterslittletous.但是，随着语言的发展，if与or，ornot连用也已为人们所接受。如：Hedidn’tknowwhether/ifweshou