Is said whole, with nothing composed onto it and takes no だ／です, so there are no forms to conjugate.
いいえ、結構です
iiekekkoudesuno, thank you — I'm fine as it isexpression
Example sentences2
いいえ、結構です。もう十分いただきました。
iie, kekkou desu. mou juubun itadakimashita.
No, thank you — I've had plenty already. neutral
いいえ、結構です。一人で持てますので。
iie, kekkou desu. hitori de motemasu node.
No, thank you — I can manage it myself. neutral