なくてはならない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.

ねばならない and なくてはならない both express obligation or necessity, meaning something 'must' or 'needs' to be done. They are equivalent in meaning, but ねばならない is a highly formal expression used primarily in the written language and in historical speech. Note that in colloquial everyday conversation, なくてはならない is often shortened to なくちゃ.

なくちゃ, なきゃ, and なくてはならない express obligation, meaning something 'must' or 'needs' to be done. なくてはならない is more formal, while なくちゃ・なきゃ (shortened forms of なくては and the similar なければ) are casual alternatives. なくてはならない is often seen in writing or official statements, while なくちゃ・なきゃ would be used colloquially with friends and family.

なくてはいけない and なくてはならない express obligation or necessity, meaning something 'must' or 'needs to' be done. They are equivalent in meaning, but ならない is slightly more formal and more common in written language, such as rules or regulations. In casual speech, both forms are often shortened to なくちゃ, with the いけない or ならない part left implied.

なくてはならない and なければいけない express obligation, meaning something 'must' or 'needs' to be done. The two forms are equivalent. The level of formality is similar, with なければ being slightly more formal than なくては and ならない being slightly more formal than いけない. The parts are interchangeable, and in casual speech, these forms are often shortened to なくちゃ and なきゃ, respectively.

てはいけない expresses prohibition, meaning something 'must not' or 'cannot' be done. In contrast, なくてはならない expresses obligation or something that 'must' be done. These forms can be tricky for learners, as the negative form なくてはならない (literally, 'it's not good if you don't') means 'must,' while the affirmative form てはいけない ('it's not good if you do') means 'must not.'

Both ないと and なくてはならない both express obligation or necessity, meaning something 'must' or 'needs' to be done. なくてはならない is more formal, while ないと (negative form with a condition と) is a more casual alternative. While ないと can be used alone or followed by either いけない (more formal) or だめ (more casual), it's generally not used with the more formal ならない.