if 改为whether. whether和if常用来引导宾语从句,这时两者的含义区别很小,一般可通用.例如: ①I don't know whether/if they will come to help us.我不知道他们是否来帮助我们. ②I am not sure whether/if I'll have time to go with you.我很难说我们是否有时间跟你们一起去. 上面两句无区别.但是,当whether与or not连成一个词组时,whether不可换用if.例如: I don't know whether or not they will come for our help.我不知道他们是否要来求我们支援.