What — a pronoun used to ask about things, actions, or information.
これは何ですか？
What is this?
明日何をしますか。
What will you do tomorrow?
何が原因で遅れたのか教えてください。
Please tell me what caused the delay.
A placeholder meaning 'that thing / you-know-what' used to refer vaguely to something known to speaker and listener.
昨日の、何あれ見た？
Did you see that thing yesterday?
あの人の、何あのやつがまた来たよ。
That person's—you know—thing came again.
A filler for a name or word you can't recall — 'whatsit / what's-his-name / what's-her-name.'
あの何、ほら、昔の映画に出てた俳優の名前は何だっけ？
That, what's his name—the actor who was in that old movie?
何ていうか、あの人、親切だったよね。
What's-it-called, that person was kind, right?
Colloquial/crude slang 'nani' (ナニ) referring to the penis; used informally or jokingly.
子どもが『ナニって何？』と聞いたので、親が適切に説明した。
A child asked 'What's a 'nani'?', so the parent explained it appropriately.
Adverbial use in negative constructions (e.g. '何とも〜ない') meaning 'not at all' or 'cannot really (say/feel)'.
その件については何とも言えない。
I can't really say anything about that matter.
正直、何とも言えない気持ちだ。
Honestly, I have feelings I can't really describe.
Interjection 'what?' or 'huh?' expressing surprise or asking for repetition.
何!? 本当なのか？
What!? Is that true?
何だって？もう一度言ってくれ。
What did you say? Say it again.
Interjection used to express anger or irritation — 'hey!/come on!/what are you saying!'.
何言ってるんだよ、冗談じゃないよ。
What are you talking about? This isn't a joke.
何だよ、それはおかしいだろう！
What the hell, that's ridiculous!
Interjection used to dismiss or minimize someone's worry — 'Oh, it's nothing; don't worry about it.'
何、気にすることないよ。次はうまくいくよ。
Don't worry about it. It'll go well next time.
『ごめん』—『何、いいよ』
'Sorry.' — 'It's nothing, it's fine.'