Negation changes the Accusative case to Genitive :) All the other cases stay the same when negated.
"I do not feel ache" probably shouldn't be accepted. Perhaps "I do not feel an ache"?
You are correct. A noun needs an article when there is no other noun determiner used.
It's "ó kreskowanE", although I don't know how to explain why.
Well... It's hard for me to say how often ó changes into o... for me it's obvious that it doesn't do it here, but then again natives usually don't think of any rules, they just know how to use the language ;)
Does this mean "I don't feel pain at this moment", "I don't feel pain ever", or either?
I wrote "I'm not in pain" and it was rejected.