1An interjection expressing a sudden welling of emotion — wonder, sorrow, longing, or dawning realization tied to feeling. As in 「ああ、きれい」(Ah, how pretty) or 「ああ、そうだったのか」(Ah, so that was it). In literary writing it is sometimes spelled 嗚呼 or 噫, but in modern Japanese it is essentially always written in kana.
ああ、なんてきれいな夕日だろう。
Ah, what a beautiful sunset.
ああ、あの頃が懐かしい。
Ah, I miss those days.
ああ、それは残念だったね。
Oh, that's too bad.
ああ、ようやく週末だ。
Ah, the weekend's finally here.
2An affirmative response equivalent to 'yes' or 'yeah' — close to はい / うん in meaning, but with a slightly gruff, masculine tone. Common between family, friends, or peers, and generally not used toward superiors.
「お茶でも飲む？」「ああ、ありがとう。」
「Want some tea?」 「Yeah, thanks.」
「もう食べた？」「ああ、さっき食べたよ。」
「Have you eaten?」 「Yeah, I ate a while ago.」
「明日も来るんだよね？」「ああ、行くよ。」
「You're coming tomorrow too, right?」 「Yeah, I'll be there.」
3An interjection of frustration, exasperation, or dismay — close to English 'argh' or 'aah'. Bursts out at moments of failure or when things don't go to plan, as in 「ああ、もう！」(Argh, come on!).
ああ、もう！また鍵を忘れた。
Argh! I forgot my keys again.
ああ、間に合わない！
Argh, I'm not going to make it!
ああ、なんでこんなことになったんだ。
Aah, how did it end up like this?
4A call to attract someone's attention, close to 'hey' or 'yo'. Less common in modern conversation than おい or ねえ — encountered more often in novels and scripted dialogue. Sounds blunt in spoken use.
ああ、君、ちょっとこっちへ来てくれないか。
Hey you, would you come here for a moment?
ああ、そこの人、忘れ物ですよ。
Hey, you over there — you forgot something.
5A light backchannel/acknowledgement uttered while listening — close to 'uh-huh', 'yeah yeah', or 'right'. Functions as a discourse marker showing comprehension or agreement without the emotional charge of sense 1. Repeated 「ああ、ああ」 carries the nuance of 'yeah yeah, got it'.
ああ、なるほど、そういうことね。
Ah, I see — so that's what you mean.
「鍵、ちゃんと閉めた？」「ああ、ああ、閉めたよ。」
「Did you lock up properly?」 「Yeah yeah, I locked it.」
ああ、知ってる知ってる、あの店でしょう？
Right, right, I know — that shop, yeah?