The number of people in the English speaking world who distinguish in pronunciation between witch and which, wine and whine, wen and when is relatively small, and those who make this distinction do not say w-hitch, w-hine, and w-hen, but rather hwitch, hwine, and hwen. What follows is a breathtaking story of the hw-sound and the wh-spelling.
All the Old Germanic languages had words beginning with hl-, hn-, hr-, and hw-. At first, their initial h- was pronounced like ch in Scots loch or in German ach. Very early, thirteen or fourteen centuries ago, the fricatives, that is, f, s, th, and ch (the latter as in loch) began to weaken, and ch was hit the hardest: it turned into a sound we now hear in hope and help (kindly note the alliteration in the preceding part of the sentence). As time went on, h tended to disappear altogether. In English it no longer occurs between vowels. Word initially people drop their h’s “like a house on fire.” Before l, m, and n it was also lost, and, thank heavens, spelling retains no trace of it. As a result, modern speakers do not realize that listen, neck, and roof go back to hlystan, hnecca, and hrof (nor do they need to be informed about the previous stages of their language). By contrast, wrong, wreak, and their likes—at one time they began with “real” w—have preserved this venerable ruin in spelling. The same holds for knock and gnaw, in which k- and g- have been mute for centuries.
In the Germanic group of languages, only Icelandic still has a semblance of hl-, hn-, and hr- in pronunciation. Why fricatives began to weaken, why they were sometimes dropped and sometimes hung on, and why the position before l, n, and r turned out to be more vulnerable than before vowels (compare house and help with listen, neck, and roof) are important and interesting questions studied by historical phonetics, but discussing them here would take us too far afield. One of the mysteries of this process is that h- managed to stay before w long after it was shed before l, n, and r. In Icelandic, it was even reinforced: in one of the two main dialects of that language, hw– (by that time hv-) became kv-. A similar process took place in northern Middle English, as evidenced by the use of qu-, quh-, and cu- for hw- in multiple manuscripts. In southern English, including London, hw- and w- merged eventually, but in the north and in some central regions words like which and witch remained distinct. English words with the historical initial hw– are not too many; yet the highly frequent which, when, what, why, and where (who stands somewhat apart from this group) do not allow us to forget them. Why then do we write when rather than hwen?
Above I said that in southern English hw and w merged. This is true, but it should not be understood as meaning that one day h was lost before w and that where (to give a random example) suddenly became a homophone of ware. In the history of language nothing happens suddenly. As far as we can judge, h- stopped being an independent unit in the group hw- but did not disappear, for it devoiced its neighbor w. Although voiceless l, m, n, r, and w may be hard to imagine, they exist (the first four of them are part of the sound system of Modern Icelandic), and one can produce them by saying l and the rest in a prolonged whisper. Apparently, it was this devoiced, “whispered” w for which Middle English scribes, under French influence, invented the spelling wh-. Before o (as in who), an h-like sound must have been audible and stable. Devoiced and voiced w- in where and so forth coexisted for a long time. In London, the voiced sound won out (hence our standard pronunciatioin). In other places, the coexistence continues.
Those who defend the vagaries of modern English spelling always say that by looking at an English word one can learn its history. They cannot explain why it is so important to remain forever in the past on paper (for our generation of computer writers paper is of course a mere figure of speech) if the pronunciation has altered the sound shape of words beyond recognition. Instead of indulging in fruitless polemic (in such matters, as in most others, polemic is usually a waste of time), I will go over some of the modern wh-words, so that historicity or the lack thereof may become more obvious. The numbers in parentheses refer to the century in which the word was first attested. Whack (18; a possible alteration of thwack, just as whittle  is a variant of thwittle), wheedle (17; perhaps from German, in which wedeln “wag the tail” has no h), whelk (15; Old Engl. weoloc), wherry “barge” (15; a word of unknown origin), Whig (17), and whiff (16; in sound imitating words like whew, whish, whist “a call for silence,” whoa, whoosh, whiz, and whiff, wh- may be an attempt to render the aural impression produced by a stream of air), and whit (15, as in not a whit). Consequently, modern wh– does not provide a reliable clue to those words’ origin. In while, white, wheat (which is related to white), whet, whether, whelp, whey, and some other words (“the interrogative class” is taken for granted), wh- represents Old Engl. hw- and is unobjectionable, at least from the point of view of those who distinguish whey from way, while from wile, and whale from wail.
Why should whist, the name of a card game that has been known only since the 17th century, be spelled with wh-? The etymology of the name is obscure. The game was first called whisk, perhaps from the verb whisk (from the action of whisking away the tricks), but this is guesswork, and other equally shaky conjectures have been offered. Unlike whisk, whore has a reputable etymology, going all the way to Indo-European (among its cognates are Latin carus and Old Irish cara “friend”); Goths called a prostitute hors. The word never had w-. In similar fashion, whole goes back to Old Engl. hal (with a long vowel), and w- lacks historical justification in it. An amusing word is whinyard “a short sword” (15). It has the same “ending” as daggard and poniard, but the root, though opaque, is still spelled with an h. In the 15th and the 16th century the spelling with wh- seems to have become fashionable, and all kinds of literate whippersnappers introduced it for no weighty reason, except that medieval scribes had a strong conviction that a word looks more dignified when it is long. This post could have had the title “Wh– As a Status Symbol.”