这三个连词的区别是that连接陈述句,而whether或if连接没有特殊疑问词的疑问句。
如:I said,"He will come." 改成宾语从句是I said that he will come. 
而I asked,"Will he come?" 改成宾语从句则是I asked if/whether he would come.
因此that与if, whether不同。
这三个连词的区别是that连接陈述句,而whether或if连接没有特殊疑问词的疑问句。
如:I said,"He will come." 改成宾语从句是I said that he will come. 
而I asked,"Will he come?" 改成宾语从句则是I asked if/whether he would come.
因此that与if, whether不同。