In English,
もう is regularly translated to ‘already’, or ‘anymore’, depending on if it is used in a positive-past (already), or negative (anymore) sentence. The reason that Japanese only requires one word for both of these nuances is due to
もう literally meaning ‘something has unexpectedly ended’.
-
朝（あさ）ごはんはもう食（た）べた。
I already ate breakfast. (Although you probably didn’t expect it, I have already eaten breakfast)
-
もうお金（かね）がない。
I don’t have any more money. (Although you probably didn’t expect it, I have no money)
From these sentences we can see that
もう can be translated in each case as the following.
(Past) - Although expected to not be done, it is. ‘Already’, in English.
(Negative) - Although expected to still be going, It isn’t. ‘Anymore’, in English.
This is the opposite of
まだ, which means ‘something is unexpectedly still going’.
もう may also be used to ask questions. This is when the speaker thinks that something ‘might’ have ended, but wants to confirm.
もう is often used simply as an expression of exasperation (giving up on arguing), to show that someone has already reached the point of not wanting to continue.