鋭い and 鈍い: knife blades, questions, and responses
Compare sharp and dull blades with a pointed question and a slow response in 鋭い and 鈍い. Identify the particular quality without turning it into a correct answer, completed understanding, or a judgment of overall ability.
A sharp blade and a knife that will not cut a tomato
鋭い (するどい) can describe a sharp blade. 「この包丁は刃が鋭いので扱いに注意して。」 means “This kitchen knife has a sharp blade, so handle it carefully.” The blade's sharpness is the reason for the request about handling the knife.
鈍い (にぶい) can describe a dull blade. 「この包丁は刃が鈍くて、トマトが切れない。」 means “This kitchen knife is dull and will not cut the tomato.” The blade's dullness is connected to its inability to cut the tomato. The subject is a tool's function, not a person's response.
A question that reaches the issue's heart and a slow response
「記者から問題の核心を突く鋭い質問が出た。」 means “A reporter asked a keen question that went straight to the heart of the issue.” 鋭い describes the question, and 核心を突く makes its focus concrete. A pointed question is not itself a correct answer or a victory in a dispute.
「問いかけへの反応が鈍かったので、もう一度ゆっくり説明した。」 means “The response to the question was slow, so I explained it again carefully.” 鈍かった describes the response to that question. Another explanation follows, but the sentence does not say that understanding was then achieved.
Keep the judgment tied to its particular subject
For a blade, 鋭い and 鈍い describe sharpness and dullness. For a question, 鋭い concerns reaching the core of an issue; for a response, 鈍い concerns its slowness. Identify whether the subject is 刃, 質問, or 反応 before assigning the adjective's sense.
One pointed question does not prove a correct answer or a person's entire ability. One slow response likewise does not establish intelligence or a health condition. Read the specific question, response, and repeated explanation without expanding them into a judgment about the whole person.
What qualities do the reporter's 「鋭い質問」 and the blade's 「鋭い」 describe?
Show the answer and why
The first describes a question reaching the heart of the issue; the second describes a sharp blade. A pointed question alone does not establish a correct answer or the questioner's overall ability.