Hi Tereza, Sometimes, they can be used interchangeably. Sometimes, only one of them should be used. For example, "if" is used in conditional sentences while "whether" is used to express the idea of two possibilities. Example (where "if" is preferred): "If I practise, I will improve." Here, the condition of constant practice must be satisfied before I can see any improvement. It is inappropriate to use "whether" here (Whether I practise, I will improve). Example (where "whether" is preferred): "Let John know whether you can meet his deadline." Here, there are two possibilities: you can meet his deadline OR you cannot meet his deadline. Either way, you have to inform John. Other examples: (1) "Let John know if you can't meet his deadline." Here, you only need to inform John in the event that you cannot meet his deadline. // (2) "Let John know if you can meet his deadline." Here, "if" expresses the same meaning as "whether" (two alternatives). // (3) "Let John know whether you can't meet his deadline". No one writes in this way as "whether" already expresses both positive and negative alternatives. Both "if" and "whether" can also be used interchangeably in reported speech to express two alternatives. Example: Mom asked, "Can you come home earlier today?" (direct speech) Mom asked if/whether I could come home earlier that day. (reported speech) Take note of the expression "whether or not". It is used to express the idea that it doesn't matter which of two alternatives possibilities or states is true as the outcome will be the same. Example: We are going outdoors if it rains. (If it does not rain, we will stay indoors.) We are going outdoors whether or not it rains. (We will go outdoors, and we don't care if it rains or not.) In a nutshell, use "if" if you are writing conditional sentences; use "whether" to express two alternatives.

It's hard to tell why the correction you mentioned was made, maybe more context is needed, yet if we are talking about a part of an indirect question, we can use either whether or if: I'm not sure if/whether I want to marry him. If your sentence is a part of a conditional, then only "if" could be used: If I want to marry him, I will marry him. Some more explanations: "Whether" is 1) used when talking about a choice you have to make or about something that is not certain: Maurice asked me whether I needed any help. There were times when I wondered whether or not we would get there. 2) used to say that something definitely will or will not happen whatever the situation is I'm sure we'll see each other again soon, whether here or in New York. I’m calling the doctor, whether you like it or not. whether to do something: She was uncertain whether to stay or leave Whether and if can be used interchangeably in indirect questions: He asked me if/whether I liked the house. Do you know if/whether they accept cards? and in the sentences with "I wonder if/whether": Mostly in polite requests: I wonder whether/ if you could help me. To express doubts and to make suppositions: I wondered whether/if she considered me a friend.

Quick answer is that they are often interchangeable. I don't know the grammar rule so I'll try to work it out with examples. We can go to the beach, if you want. (means: we will only go if you also want to go) We are going to the beach whether you want to or not. (means we are going for sure and you can choose to go or stay) I didn't know if/whether you wanted the beef or chicken. (both are correct) We will eat at 7:00 whether they have arrived or not. (both would work here too but whether sounds better) ********** I think whether needs an "or" statement that follows. If I agree to marry him, my mother will be pleased. (whether is incorrect here) Whether I agree to marry him or not, I'm keeping this apartment. (if or whether is fine here but whether sounds better)