まいFirm intention not to act
まい expresses a firm decision not to perform an action: “I will not” or “I am determined not to.” It has a formal, literary, or solemn tone.
A settled decision
まい presents the action as something the agent has chosen not to do. A controllable action makes this reading especially clear. It often appears before と決める, と決意する, or と誓う, which explicitly frame the words as a decision or vow.
Direct and reported resolve
私は二度と同じ過ちを繰り返すまいと心に決めた。
I resolved never to repeat the same mistake again.
AnalyzeThe following words explicitly identify this as the speaker's decision, not a prediction.
どんな圧力を受けても、信念を曲げまい。
Whatever pressure I face, I will not compromise my convictions.
AnalyzeA solemn first-person declaration. The subject is naturally understood from the declaration.
彼はもう誰にも頼るまいと決意していた。
He had resolved not to rely on anyone anymore.
AnalyzeNarration can report another person's resolve.
Decision or prediction?
The same ending can instead mean that an event is unlikely to happen. Look for agency and context: a decision, vow, or controllable refusal gives the negative-resolve reading, while an assessment of future likelihood gives the prediction reading. An animate subject alone does not settle the issue.
彼はもうここへは戻るまい。
He probably will not return here again.
AnalyzeThis is the neighboring prediction use: the speaker assesses what he is likely to do rather than reporting his decision.
More examples
私は責任から逃げまいと、自分に言い聞かせた。
I told myself that I would not run away from my responsibility.
Analyze彼女は二度と言い訳をするまいと決めていた。
She had decided never to make excuses again.
Analyze何があっても、この秘密を漏らすまいと誓った。
I swore that, whatever happened, I would not reveal this secret.
Analyze今後はこの件に口出しすまい。
From now on, I will not interfere in this matter.
Analyze彼は最後まで仲間を見捨てまいと誓った。
He vowed not to abandon his companions, no matter what.
Analyzeこの記録だけは、決して捨てまい。
This record, at least, I will never discard.
Analyze