- If Cá is followed by a noun or adjective that begins with a vowel, then that word gets a prefixed H, e.g. Cá haois í? (“What age is she?”), Cá hard é? (“How high is it?”).
- In meaning, they’re practically synonymous. At most, the difference between cá háit and cén áit is the difference between “what place” and “which place”.
The linguistic term is h-prothesis (similarly, the t- is called a t-prothesis. "Prothesis" is really the more correct term over "prefix" as a prefix would change the word' s meaning (like turn and return, changed and unchanged, and so on), while a prothesis does not change the word's meaning.
It should be. Although modern conventions try to tell you that you shouldn't have a preposition at the end of a sentence, it is only because some fool thought we should apply Latin rules to a Germanic tongue. It is entirely acceptable, and proper, in all other Germanic tongues as far as I have seen.