In sentences like "I don't know whether it will rain today," or "I don't know if it will rain today," there is no difference in meaning. The difference is one of register: using "whether" makes the sentence sound more formal than using "if" does, and, due to this difference in level of formality, we tend to find "whether" used more in writing than in speech (although you will hear "whether" in speech - it will depend on things like the speaker's educational background, what he or she is talking about, who he or she is talking to...) "whether...", "whether or not..." and "whether... or not" are all acceptable. I don't know if there are objections to "if... or not", but it is certainly used. However, *"if or not...", for example, *"I don't know if or not she will come today," is incorrect.

if you're going to use "...or not" then only use whether (its simpler and you'll sound smarter). "If" is usually used with THEN. "If it will rain to day...then ...I will stay home" "Whether" also implies a second choice (ie whether...or...) where "if" focues on only one option. That said Vivi, you'll likely find that small words like if, the, an, for etc will be the hardest to learn because they are used in so many different ways. I'd strongly recommend to just learn how to say the sentence you're interested in. Over time, you'll just get used to what sounds right and what doesn't. Hope this helps.