The edge of this wooden chair is a bit sharp—it's quite painful to sit on.

painful 是形容词，由名词 pain（疼痛）加上后缀 -ful（充满...的）构成，意思是"引起疼痛的、令人痛苦的"。这里形容坐在锋利的椅子边缘会让人感到疼痛。