'To whom do you wish to speak?" is fairly easy to grasp. But is it "Who do you want to talk to?" or "Whom do you want to talk to?"?

Because it’s semantically irrelevant.

Perhaps someone can answer this without opinion, but to provide my own speculation, I might say that as "whom" gets used less frequently in casual conversation, people aren't as accustomed to the 'right' and "wrong" way. If I were to generalize, I'd say only people who speak in a more formal way use "whom" these days.

@Jim - thanks for the quick reply, but why does its being semantically irrelevant make it difficult for a learner?

@Tom22 You are right. Some eminent grammarians including our John Lawler have actually recommended to use 'who' instead of 'whom' in almost all cases. NOTE 2: please see my question's last line added by edit: Nor is it 'primarily opinion based' if you can quote standard references to support your answer. However every user who thinks they have a reason can express their opinion in a comment!

It doesn’t make if difficult it makes it unnecessary. And because it’s unnecessary, it isn’t learned and internalized at a young age - There’s no incentive. It only gets taught at school to a subset of the population based on rules using parts of speech definitions that are equally foreign to young native speakers. And at that point it’s harder to break a bad habit than form a new one.

@Jim that is illuminating. It seems a learner is usually bothered about who and whom only when they are ready to improve their English! Luckily it's irrelevant in these modern times.

@Hot Licks somebody taught us that 'who' is linked to subject and 'whom' to object -- this was a long time back in the late 1980's -- do you think that's correct, and if so, a good rule of thumb; and is it useful to keep it clear in the minds of learners?

Where it gets confusing is when you are three layers deep in prepositional phrases.

@Hot Licks Very true. Somebody also said "when in doubt, use 'who' -- it looks much less awkward when used in a strictly incorrect place than 'whom', whose mistaken use is much more prominent to the grammatically aware reader."

Thanks to @Mari-lou A for the nice and subtle formatting -- how do you substitute the 'curved' style of inverted commas for the 'straight quotes'?

Right now my energy is somewhat depleted and focused on a different answer, but I once read a linguist suggest that "whom" is likely on its way to obsolescence, and that part of this process involves confusion among English learners. I don't have access to that original suggestion, but this article in Economist might provide a seed for someone's answer: [Is "whom" history? From the mouths of babes: How is a child to learn "whom" when adults hardly use it?] -- (economist.com/blogs/johnson/2012/10/grammar)

I have a mac: To open single curly quote: alt option + right square bracket (]) = ‘ To close single curly quote: shift + alt option + right square bracket = ’ On Windows it's: ALT + 01455 and ALT + 01456. NB You have to keep ALT pressed while you tap out the sequence of numbers. EDIT: Search “curly quotes” on Google

@Mari-lou A unfortunately I am using an android phone, and my subsequent edits to the question have introduced a few more straight quotes that look dull compared to your 'curly quotes' -- I shall try to find out how to do it using an android keyboard, and edit at the earliest.

Well, speaking of formatting, that comment is irreparably embarrassing. :P I don't know why but I've always been bad at posting links in comments. Anyhow, I highly recommend the article, it's good reading.

@RaceYouAnytime thanks for the concept: it makes eminent sense that something 'on the way to obsolescence' would create confusion for learners. The crucial statistic in the excellent linked article is that 'who' is 57 times more common than 'whom'. It appears just 53 times out of every million words. If it was written right and I interpreted it right, ('who' appears 53 times in a million words) the word 'whom' has become literally one in a million!

@RaceYouAnytime the thing is that I appreciate the 'formal' register and approve of using 'whom' where it is appropriate. I cannot accept that it is good for 'who' to replace 'whom' all the way into obsolescence. 'Whim' has its iwn continuing significance, when used in the right place. As I mentioned in my answer to the earlier linked question, both language and literature would be the poorer if we were to say henceforth, "for who the bell tolls!"

I agree with you, and I don't think "whom" will disappear anytime soon. But it's also worth noting that poetic writing is still free to use archaic language; after all, "it tolls for thee."

@RaceYouAnytime You make a very good point; the article you linked is also confident that 'whom' will endure in literate/ literary circles for a long time to come.

English is generally losing inflection; we pretty much only preserve it in pronouns now, and not as much there as in centuries past (thee and thou, I'm looking at you). I don't know why that is, exactly, but there seems to be a general trend to replace that functionality with other linguistic features. So my theory is that English itself is becoming more resistant to whom and English speakers' brains are just less wired for it somehow. (Thus the common baby-talk "me do it!") I would imagine that folks who speak a more fully inflected language might find this easier.

@1006a many thanks for the explanation; English like many another language is constantly evolving and 'whom' seems to have become (not a unique, but) just another victim of the push for functionality at the expense of formal elegance. As in sports: if 'who' can cover both positions we don't need 'whom'! However I was originally asking why learning who and whom is difficult, especially for a non-native speaker?

Cont. I'm not a linguist or psychologist, so I don't know for sure, but I suspect this kind of distinction is harder because it's in some way odd for the language. The errors made by young children who are learning their own language tend to be errors of over-regularizing, because they quickly internalize the most basic rules. For example, they might say "I goed" instead of "I went" because they have internalized the basic rule for putting a verb into the past tense as "add -ed at the end" but still need to learn the exceptions. So the fact that young children tend to say "him is my friend"...

...suggests to me that the distinction between he/him, I/me, who/whom etc. is in some way not a regular feature of English, on a deeper level than prescriptive grammar.

@1006a it's very insightful of you to suggest that these are some sort of 'artificial distinctions' and possibly not integral to the fundamental design of the organic language -- in fact I was not aware that even native speakers struggle with 'who' and 'whom' -- and as you say about non-native speakers, learners are probably getting conflicting information, both in terms of examples and "rules" is exactly whst I was thinking of -- thank you for a very cogent hypothesis; would you consider combining these comments and posting it as an answer? I shall not be bothered by the lack of citations.

It's just a wild theory and I don't have time right now to research its merits, so I'd rather wait to see if one of our actual experts has an answer. Anyone else is free to take anything they want from the comments in the meantime, though.