なければならないMust do, Have to do

なくてはならない and なければならない both express obligation or necessity, meaning something 'must' or 'needs' to be done. They are equivalent in meaning, but なければならない is slightly more formal and common in written rules or official language. In casual speech, these forms are often shortened to なくちゃ and なきゃ, respectively.

Both なくちゃ・なきゃ and なければならない express obligation or necessity, meaning something 'must' or 'needs to' be done. なければならない is more formal and commonly found in writing or official statements. In contrast, なくちゃ・なきゃ are casual contractions (of なくては and なければ) used in everyday speech with friends and family.

なくてはいけない and なければいけない express obligation or necessity, saying something 'must' or 'needs to' be done. They are equivalent in meaning, but the なければならない version is slightly more formal, and thus more likely to be found in written rules, etc. Note that in casual speech, these forms are often shortened to なくちゃ and なきゃ, respectively.

Both express obligation with a nuance of rule or necessity, e.g. 行かねばならない and 行かなければならない both mean 'must go'. なければならない is formal; ねばならない is even more formal and often appears in academic or historical texts. In conversation, contractions like なきゃ are more common.

These are opposites: なければならない 'must' vs. べきではない 'should not'. The former suggests a rule or necessity; the latter implies social or moral advice, e.g. 行かなければならない ('You must go.') as compared to 行くべきではない ('You ought not to / shouldn't go.').

てはいけない is used to express prohibition, meaning that something 'must not' or 'cannot' be done. なければならない expresses the opposite meaning, i.e., that something 'must' be done. These forms can be tricky for learners, as なければならない appears negative but means 'must' (literally, 'it's not good if you don't'), while てはいけない appears affirmative but means 'must not' (literally, 'it's not good if you do'). (いけない and ならない themselves are essentially synonymous, though ならない is somewhat more formal/written-style.)

These express opposites: なければならない 'must' vs. てはならない 'must not'. For example, 行かなければならない 'must go' is the opposite of 行ってはならない 'must not go'. Both are formal and typically used in writing or structured speech. Casual speech favors shorter forms like なきゃ and ちゃ.

These two patterns both express obligation (i.e. 'must do' or 'have to do'). For example, 行かなければならない and 行かなければいけない both express an obligation to go, but the ならない version is slightly more formal and found more often in writing. In everyday conversation, contractions like なきゃ (e.g. 行かなきゃ) are more common.

Both express obligation, but differ in tone. ないと is casual and often shortened in speech, as in 行かないと ('I've got to go.'). なければならない is more formal, used in writing or polite conversation. Both convey the idea of needing to do something.

ことだ is used for light advice ('should' or 'ought to'), e.g. 毎日早起きすることだ ('You should get up early every day.'). なければならない expresses stronger obligation, like 'must' or 'have to', often implying consequences. ７時に起きなければならない ('You must get up at 7.').