That's the thing about language: it's pretty complicated. (This is something I discovered on my own. I am brilliant that way.) People are complicated, the things they might wish to express are complicated, and their social environment is complicated. These things conspire to render language complicated. I'm convinced that many nuances of Korean will be forever beyond me, even after (if?) I become more proficient. For instance, the difference between -는지 and -냐고 in sentences like 공부한 학생이 시험을 잘 볼건지 물어봤어요 ("He asked whether a student who studies will do well on the test") and 공부한 학생이 시험을 잘 볼거냐고 물어봤어요 ("He asked if a student who studies will do well on the test").
At least, these are the different (?) meanings my teacher would assign to those sentences. Other Koreans have told me the sentences feel different but in ways that are very difficult to express. Which is fair. Lots of these kinds of nuances are very hard to sort out and explain. (I've experienced this many times when I'm working with English language learners. They will ask how these two words or constructions differ, and sometimes, even if I'm absolutely certain they do differ, I really can't say how they differ.) So I'm not uncomfortable when I encounter things like this in Korean. I just toss them on the pile of things I don't really understand, halfheartedly resolving to revisit them later. But this case. I'm not sure.
Because I don't feel any difference in those two English sentences, the one with whether and the one with if. There are certainly some cases where the choice of whether or if affects the meaning. (And I'm aware that proper/standard/fussy usage might rule against if and in favor of whether in some other cases.)
Compare:
A. Let me know if you need to borrow my chainsaw.
B. Let me know whether you need to borrow my chainsaw.
A could be paraphrased as "If in fact you do need to borrow my chainsaw, let me know," while B could be paraphrased as "Let me know—one way or the other—about your need to borrow my chainsaw."
Two very different meanings. (Of course, yes, sometimes they're used in exactly the same ways.)
But in sentences like this, I don't hear any difference:
C. The cashier asked if I needed a receipt.
D. The cashier asked whether I needed a receipt.
I think C and D mean exactly the same thing. (If I were editing C—and the context wasn't informal or loose—I would probably "convert" it to D. So, yes, there might be some stylistic difference, but I don't think there's any semantic or logical difference.)
Which brings me back to the Korean distinction, supposedly a whether/if distinction. Basically, I'm not buying it. There might, in fact, be some nuance there, but I don't think resorting to whether and if will help us make sense of it. I think Korean speakers just have a hard time formulating and expressing the nature of this distinction, so it might just remain invisible to me for a while (forever). It reminds me of my attempts to explain the difference between will and might to Koreans. Apparently, it's not something Korean captures neatly. Although, now that I think of it, it seems much clearer than this supposed whether/if business.
