1An adverb meaning "already" (an action or state has been realized) or "(not) anymore" (a state no longer continues); often used as 「もう〜した」 or 「もう〜ない」.
もうご飯を食べましたか。
Have you already eaten?
彼はもうここにはいません。
He is no longer here.
その映画はもう終わった。
That movie has already ended.
もう子どもじゃないんだから、自分で決めなさい。
You're not a kid anymore, so decide for yourself.
2An adverb meaning "soon" or "shortly," indicating that something will happen in the near future; often used as 「もうすぐ」.
もうすぐ電車が来ます。
The train will be here soon.
もう春がやってくる。
Spring is coming soon.
あと少しでもう着きますよ。
We'll arrive very soon.
3An adverb meaning "more," "further," or "again," used to add to an existing amount or repeat an action (e.g. 「もう一つ」 "one more").
もう一杯お茶をいただけますか。
Could I have one more cup of tea?
もう少し大きい声で話してください。
Please speak a little louder.
もう一度、最初から説明します。
I'll explain from the beginning once more.
4An interjection used to emphasize frustration, surprise, or exasperation — like "jeez" or "come on"; typically used casually on its own as 「もう！」.
もう！何度言えばわかるの？
Jeez! How many times do I have to tell you?
もう、また遅刻したの？
Come on, you're late again?