ねばならない

1. 1Must or have to do something as a duty, principle, or necessity.Ex. 1
2. 2Must do something because of a prior circumstance or commitment, often after 以上 or からには.Ex. 2

Written and formal register, typical of essays, editorials, speeches, and statements of principle or resolve. It carries a firm, somewhat solemn tone and sounds stiff in everyday conversation, where なければならない or なきゃ is preferred. It often follows 以上 or からには to state an obligation that follows from a situation.

• なければいけない / なければならないN4「なければいけない/ならない」 has essentially the same meaning, and the two are often interchangeable in writing; however, it is neutral in register and common in everyday conversation. For conjugation, する becomes 「しなければ」, and nouns take 「でなければ」.
• なくてはいけない / なくてはならないN4「なくてはいけない／ならない」 has essentially the same meaning but is neutral in register, usable in both conversation and writing; in speech it can be contracted to 「なくちゃ」. It conjugates as 「書かなくては」, with nouns taking 「でなくては」.
• ざるを得ないN2「ざるを得ない」 emphasizes having no choice due to circumstances and usually carries a sense of resignation; it can also attach to verbs like 「言う」 or 「認める」 to mean 'cannot help but admit'. 「ねばならない」 focuses on obligation or what ought to be, and can express resolve. Both are formal, and the verb conjugation is the same.
• べきだN2「べきだ」 attaches to the dictionary form of a verb and expresses the speaker's assertion that something 'should' be done by reason, with a neutral register; 「べきだった」 can express regret. This pattern attaches to the ない-form minus ない and means 'must', in a more formal style. When saying something 'ought to be so', the two are often interchangeable.
• 必要があるN4「必要がある」 objectively states that something is 'necessary', with a weak sense of compulsion and a neutral register; it attaches to the dictionary form of a verb or to a noun plus の. It can be replaced only in the sense of 'must do something', making the tone softer; it should not be substituted in the 'ought to be so' usage.

ねばならない means the same as なければならない/なくてはならない but is more literary, whereas those are neutral and common in speech; 必要がある states an objective need rather than a firm duty. べきだ expresses what one ought to do by moral judgment or advisability, while ざるを得ない implies reluctantly having no other choice, a nuance ねばならない does not carry.