"You are eating supper."
Thanks for neither specifying plural or singular nor which word for supper you were looking for. I was wrong on both counts.
Without context, both singular and plural (jesz and jecie) need to be accepted, but about supper, I think it only translates to kolacja (the last of the three most important meals), it is obiad which has the neverstopping lunch/dinner debate.
What was your version?
So how do Polish speakers know whether the person means lunch or dinner when they say obiad? Or does the speaker have to indicate which meal they're talking about before?
That's the case, for Polish people everything is obvious. It's English that is confusing ;) At least across dialects.
If you consider the tree 'main meals' of the day, obiad is the second one and kolacja the third one.