> what is difference between "c" and "k" in English language
They're the same sound, including when it's written "ck" as in "back". Although sometimes the "c" is pronounced like an "s", e.g. "face", "celery".
English spelling is highly inconsistent and it's often hard to tell how a word should be pronounced based on the spelling alone. Don't expect it to make sense.
It only makes sense if you know how the word entered the english language. Face and celery comes from french which pronounces these words with a soft c. So we pronounce it with a soft c. Coupe comes from french which pronounces it with a hard c. Hence why we pronounce coupe with a hard c. The 'strange' spelling and pronunciation of words with c is a result of loan words from many languages ( french, latin, german, spanish, etc ).
> Is the etymology really needed in this case to know how to pronounce "c"?
No. You learn it through osmosis by being immersed in the culture. We all learn to pronounce face, celery, coupe, etc from our parents, school, media, etc. Etymology only comes into play if you want to know why we pronounce it that way.
> I believe in -ce- -ci- -cy- the c is an s, while in -ca- -co- -cu- the c is a k.
That probably works most of the time since I believe that's the rule the french use and so much of our vocabulary came from the french. But there are exceptions as others have noted. 'Boston Celtics' is pronounced with a soft c, while the 'celtic people', is pronounced with a hard c. As far as I know it's cultural. There isn't a rule which will help you out here other than you simply have to know it.
The links in Everything2 go to the correct word if you're having difficulty.
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A Plan for the Improvement of English Spelling
For example, in Year 1 that useless letter "c" would be dropped to be replased either by "k" or "s", and likewise "x" would no longer be part of the alphabet. The only kase in which "c" would be retained would be the "ch" formation, which will be dealt with later. Year 2 might reform "w" spelling, so that "which" and "one" would take the same konsonant, wile Year 3 might well abolish "y" replasing it with "i" and Iear 4 might fiks the "g/j" anomali wonse and for all.
Jenerally, then, the improvement would kontinue iear bai iear with Iear 5 doing awai with useless double konsonants, and Iears 6-12 or so modifaiing vowlz and the rimeining voist and unvoist konsonants. Bai Iear 15 or sou, it wud fainali bi posibl tu meik ius ov thi ridandant letez "c", "y" and "x" -- bai now jast a memori in the maindz ov ould doderez -- tu riplais "ch", "sh", and "th" rispektivli.
Fainali, xen, aafte sam 20 iers ov orxogrefkl riform, wi wud hev a lojikl, kohirnt speling in ius xrewawt xe Ingliy-spiking werld.
I find it fascinating that the writer uses “aafte” for “after.” Clearly their accent was non-rhotic, which isn’t something we’d have any idea about otherwise.
Spelling does reflect pronunciation. The attempt to standardize spelling is rather new, though. It's actually more surprising that there should be canonical spellings when the pronunciation itself varies between speakers and dialects, and it changes over time.
Dialectal variety is true for many other languages, though, which nevertheless manage to do some kind of standardized spelling.
Thing is, in most cases when it's claimed that "spelling reflects pronunciation" for some language, it's not actually true - what it reflects is the phonemes of the language, not phones (actual sounds). To give an example in English, /t/ is a single phoneme, but it can be realized in a bunch of different ways ranging from actual [t] in words like "tea" to a glottal stop [ʔ] in words like "butter" (frequent in UK). Native speakers often don't even notice that these are two completely different sounds, because they are mentally mapped to the same underlying phoneme.
But because our brains are already perfectly capable of handling such mappings (so long as they happen according to consistent rules, which they almost always do), a phonemic spelling works great in practice. And when you deal with phonemes, the difference between various accents and dialects is usually much less than when you look at the phones.
English is problematic because its spelling is strongly inconsistent even with the phonemes, never mind the phones. And, yes, the rather extreme dialectal variety of English means that different dialects do have some phonemic variety as well. But, again, it's much less than actual pronunciation differences would make you believe, and it's certainly possible to come up with a canonical mostly-phonemic spelling that would cover, at the very least, British RP and American GA, and be reasonably adequate for most offshoots of those.
It's also possible to just adopt phonemic spelling based on specific dialects and expect the speakers to handle the differences at least for the popular ones (just like we already do with different words like "elevator" vs "lift"). This was the approach taken by Serbo-Croatian, where e.g. "vreme", "vrime", and "vrijeme" would all be valid spellings of the same word.
They're the same sound, including when it's written "ck" as in "back". Although sometimes the "c" is pronounced like an "s", e.g. "face", "celery".
English spelling is highly inconsistent and it's often hard to tell how a word should be pronounced based on the spelling alone. Don't expect it to make sense.