DO NOT TOUCH MY BUTTER!
"Is this not your butter?" should be accepted, no?
It should be and it is, it's even starred.
why is it maslo and not masla? would you not use the genitive case?
Only accusative changes to genitive, when negated. This was a nominative sentence of the type "To jest + Nominative".
I used (and accepted) 'Is this not your butter?'- a somewhat antique but lovely way of putting it as one presumably wipes it from one's jacket, it having been accidentally flicked from an adjacent table.