An informal interjection meaning 'yes' or 'yeah' used to indicate agreement.
「明日行ける？」「うん、行けるよ。」
"Can you go tomorrow?" "Yeah, I can go."
先生：宿題は出した？ 生徒：うん、出しました。
Teacher: Did you hand in the homework? Student: Yes, I did.
うん、そうだね。君の言う通りだ。
Yeah, that's right. You're right.
An interjection expressing hesitation or thinking (like 'hmm' or 'well').
うーん、それは難しいね。どうしようか。
Hmm, that's difficult. What should we do?
質問を聞いて、彼はしばらく『うん…』と考え込んだ。
Hearing the question, he paused and said 'hmm...' while thinking.
A groan or moan of pain; a short sound expressing discomfort or strain.
転んで膝を打ち、彼はうっ…うんと呻いた。
He fell and hit his knee, and gave a muffled groan.