回答如下:
它们的用法不尽相同:
if 和whether都可以引导作动词宾语的宾语从句。例如:
I wonder if / whether he will go.
if从句不能作介词宾语,而whethe从句则可以。例如:
I don't care about whether he will go.
if从句不能作表语、同位语,而whethe从句则可以。例如:
My question is whether he will go.
They are discussing the question whether they will pull down the house.
句首不能用if从句作主语,而whethe从句则可以; if / whether从句均可以置于主句后面作主语(前面主句用it作形式主语)。例如:
Whether he will go or not is not clear.
It is not clear whether / if he will go or not.