when do I use "if" and "whether"?

Is there anyone who can give me examples of those conjuctions usage because I can’t realize the difference between them.

" If for a conditional idea, whether for an alternative or possibility. Thus, Let me know if you’ll be coming means that I want to hear from you only if you’re coming. But Let me know whether you’ll be coming, means that I want to hear from you about your plans one way or the other. "

After Prepositions , we use only whether.
There was an argument about whether I should go and visit her.

[b]If cannot introduce a subject clause :[/b]

Whether she is tired is not so important.
If she is tired is not so important (incorrect)

Generally speaking, use “if” to express a condition.


Even though it’s gramatically incorrect, would it be acceptable to use “if” during an informal talk?
I mean in the situations that you have recommended NOT to use it.

thanks for your explanation Michauek.