I need to write this out to be corrected or confirmed.
The reason this sentence calls 'velcí' instead of some other form of the Czech word for big/large: The noun being described is masculine animate plural nominative, so the hard-ending-adjective requires an -í. And, because of the consonant shift required when appending an 'í', the 'k' gets converted to a 'c'. So we get 'velcí'.
Is that correct?
Yes, í cannot be used with a hard consonant (h, ch, k, r, d, t, n) and it changes to a soft consonant, so there is a regular shift.
krásný, krásní (ní means ňí)
mladý->mladí (dí means ďí)
prostý->prostí (tí means ťí)
This only happens for hard consonants, it does not happen for the group of "ambivalent" or "neutral" consonants b, f, l, m, p, s, v, z that can be used with y and i in various words.
hrubý->hrubí and so on...
Why is not the consonant before 'i' always palatal? What I hear is 'osly' in this very sentence.
Also, the word 'velcí' which sounded like 'velcy' to me made me think that there is a rule in Czech, one just like in Russian - that 'i' after c, ž, š does not make the consonant palatal. Am I right?
Czech doesn't have the old Slavic palatalized/non-palatalized distinction that is still maintained in Russian. Instead of that, we simply have a set of hard consonants: h, ch, k, r, d, t, n; a set of soft consonants: ž, š, č, ř, c, j, ď, ť, ň, and a set of ambivalent consonants: b, f, l, m, p, s, v, z.
Czech has also lost the distinction between soft and hard /L/ - we only have one /L/ and it's an "ambivalent" consonant. There is no difference in the pronunciation of "osli" (nominative) and "osly" (accusative).
The consonant before "i" must be either soft or ambivalent, whereas the consonant before "y" must be either hard or ambivalent. Whenever you put an "i" after a hard consonant, it immediately changes into a corresponding soft consonant - that's why we have "velký" (singular masc.) -> "velcí" (plural masc. anim.). But nothing happens to the /c/ in "velcí" - there is only one /c/ sound.
No, you must use the plural verb:
"Jsou to velcí osli." "To jsou velcí osli." "Oni jsou velcí osli."
These are the same sentences in singular:
"Je to velký osel." "To je velký osel." "On je velký osel."
The singular verb also works with numbers 5 and higher (with the donkeys in genitive):
"Je to pět velkých oslů."