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 ).
I believe in -ce- -ci- -cy- the c is an s, while in -ca- -co- -cu- the c is a k.
Perhaps the etymology is needed to know how to write a k/s sound you've never heard before.
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.