rahulvsd, it sounds like you've actually got a good handle on it!

Sometimes "whether" is a substitute for "if" - consider this:

"We don't know whether he's going."

You could easily say:

"We don't know if he's going."

Yes, "whether" is typically expressing two alternatives - "whether or not" is common usage, but in many cases someone could say "if" instead of that phrase. (It is confusing, isn't it??)

Here's the one instance where it's different - when "whether or not" is used to express an ABSOLUTE:

"We are going whether or not it rains."

In this case, it means that no matter what - regardless of if one case happens or the other case happens - we are definitely going.

Not sure if this is helping you, but it is so interesting to try and explain these oddities of the English language!!

EssaySnark