if作如果讲时后面主句用的是将来式或者情态动词
如：We'll go to the mountains if the weather is fine tomorrow.
if作是否讲时,后面跟的事态随主句变化,与whether可以互换
如：I don't know if he will come tomorrow.
I didn't know if he would come the day after tomorrow.
if后面跟什么时态
if作如果讲时后面主句用的是将来式或者情态动词
如：We'll go to the mountains if the weather is fine tomorrow.
if作是否讲时,后面跟的事态随主句变化,与whether可以互换
如：I don't know if he will come tomorrow.
I didn't know if he would come the day after tomorrow.