Hi Anna, Please have a look at the following info: 1. When both choices are given, we generally use whether (more common in formal writing) rather than if (more common in informal writing): I don't know whether she's coming or not. (It would be possible to use if here, but less common.) 2. Whether, (but not "IF"), is used before –to infinitives: I don't know whether to accept their offer or not. (If would not be used here.) 3. After a preposition, only whether is possible: It's a question of whether we can agree on everything. (If would not be used here.) 4. Both whether and if can be used in indirect speech to introduce a yes/no question: Did they say whether/if they were going to be late? 5. After some verbs, we use whether: I doubt whether they'll make it. We discussed whether it was the correct decision. However, "what if" is something totally different than the above, we normally use what if for suggestions; note that the main verb can be present or past, a past form makes the suggestion sound less definite. What if I CAME tomorrow instead of this afternoon? Hope that this helps and let me know if you need any further info. Thks Bro

"If" is a conditional. "What if" is usually the basis of a hypothetical. "Whether" is used when there are two present alternatives. Conversely "neither" is used as a opposite effect for two alternatives. "I don't care whether you sink or swim." "I could neither laugh nor cry."

what if it rains?要是下雨怎么办? I wonder if it's going to rain. I wonder whether it's going to rain or not. whether + or not,if 不加 or not