"To nie jest twoje masło?"

Translation: This is not your butter?

[idanlipin]

DO NOT TOUCH MY BUTTER!

[Mim_Fox]

"Is this not your butter?" should be accepted, no?

[Jellei]

It should be and it is, it's even starred.

[Marysia511745]

why is it maslo and not masla? would you not use the genitive case?

[alik1989]

Only accusative changes to genitive, when negated. This was a nominative sentence of the type "To jest + Nominative".

[Stewart288923]

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.