もう

1. 1Already: indicates that an action has been completed (usually with a past-tense verb) or that a state or point in time has been reached (もう春です).Ex. 1
2. 2No longer / not anymore: with a negative predicate, shows that a former state has ended.
3. 3More / another: before a number and counter or a degree word such as 少し, adds an extra amount.

Neutral and very common in everyday speech and writing. In questions like もう〜ましたか it asks whether something has been done yet, and with nouns or time expressions it notes that a state or point has arrived (もう春です). Unlike まだ, which expresses 'still' or 'not yet', もう marks that a point of completion or change has been reached.

• まだN5「まだ」signals that a change has not yet happened and is the opposite of 「もう」, so the two cannot be swapped. In negatives, 「まだ〜ていません」 means 'not yet' while 「もう〜ない」 means 'no longer,' and the negative answer to 「もう〜ましたか」 is 「まだです」.
• もはやN1「もはや」 sounds stiff and is mostly for written or formal contexts, covering only 「もう」's 'no longer' and 'the state now holds' uses to stress that the situation is settled. It cannot express an action's completion or 'again' — for example, 「もはやご飯を食べた」 is not said.
• てしまう / ちゃうN4「てしまう」, a supplementary verb (補助動詞) that attaches to a verb's て-form, stresses finishing completely or irreversibly and carries emotional coloring. 「もう」 in the sense of 'already' is an adverb that simply reports completion without that coloring; the two are not interchangeable but often combine, as in 「もう食べてしまった」.
• たばかりN4They are close only when 「もう」 means 'already': 「もう」 is an adverb marking a completed action or a realized state, while 「たばかり」 attaches to a verb's た-form and stresses that (subjectively) little time has passed since completion, often to give a reason. Generally they cannot be swapped.
• とっくにN2「とっくに」 stresses 'long since,' is mostly spoken, and can replace 「もう」 in its 'already' sense; it also works for a state that stopped holding long ago, as in 「とっくに子供じゃない」. It cannot be used for 'will no longer do X from now on' or for 'again / one more.'

まだ is the counterpart of もう, meaning 'still' or 'not yet'. もはや is a formal, literary word meaning 'already, by now' (もはや手遅れだ) or, with a negative, 'no longer', while とっくに stresses that something happened long ago, たばかり says an action has just been completed, and てしまう expresses completion or regret through a verb form rather than an adverb.