"I sit to that one, on which there is a pillow."
Translation:Arra ülök, amin van párna.
There is a mismatch in translation here.
"Arra ülök, amin van párna." means "I sit on whatever has a pillow on it". For example if I see a cat with a pillow on its back, I sit on it.
And "Arra ülök, amelyiken van párna." means "I sit on the one that has a pillow on it." This one could be a good answer to "Which chair do you sit on?".
And the English sentence above, well, I am not sure what it means.