Needs Must

In this Topic, we're going to learn about a variety of ways to say that someone must or must not do something. We'll learn how all these phrases come down to the same core idea, and how they vary based on formality.

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てはいけない
Must not, May not
てはいけない is used in Japanese to express that (A) 'cannot', or 'must not' be done. It is the combination of a verb in て form, the は particle, and the negative potential form of 行(い)く (to be able to go). In this way, it literally means '(A) cannot/is not able to go'.
Although は can be dropped from many phrases and expressions, the は in this particular grammar point is usually not. This is because いけない would then become the second action in a sequence of events (the usual function of て form). However, to make this more casual, ては is often changed to ちゃ (ては becomes ちゃ), or じゃ (では becomes じゃ), as this is easier to say.
Caution

Apart from いけない; ならない and だめ are also frequently used to get across the same meaning. However, they are used in different situations.

いけない - Fairly neutral. Used in both spoken and written language.

ならない - Fairly formal. Used mostly in written language.

だめ - Very casual. Used almost exclusively in spoken language.
N5 文法
なくてはいけない
Must do, Have to do
Much like てはいけない, なくてはいけない states that something 'must not' be done. However, this is where the similarities end. なくてはいけない is an example of a double negative in Japanese, and actually has the meaning '(A) must be done', or 'must do (A)'.

While this grammar point is usually translated as 'must do', the literal translation is 'must not, not do (A)'. This double 'not' is where many learners have difficulties. いけない simply means 'cannot go', and comes from the negative potential form of 行(い)く.

To use this structure, simply create the negative form of a verb, and then convert ない to なく (the conjunctive form of an い-Adjective), followed by て. After this, は is added, and then finally いけない.
いけない is considered to be the most common structure used with the 'must', and 'must not' grammar points, and is natural in both speech, and written Japanese. Alternatively, ならない may be used in formal speech, but is much better suited to writing. だめ is also acceptable, but much more casual than either いけない or ならない.
Caution

The ては in なくては is often converted to ちゃ, as this is easier to say. なくちゃ may then be followed by いけない, or いけない may be omitted, without changing the meaning.

These casual structures are very similar to words like 'gotta' in English.
N5 文法
なくてはならない
Must do, Have to do
Much like てはならない, なくてはならない states that something 'must not' be done. However, this is where the similarities end. なくてはならない is an example of a double negative in Japanese, and actually has the meaning '(A) must be done', or 'must do (A)'.

While this grammar point is usually translated as 'must do', the literal translation is 'must not, not do (A)'. This double 'not' is where many learners have difficulties. ならない simply means 'cannot become', and comes from the negative form of なる.

To use this structure, simply create the negative form of a verb, and then convert ない to なく (the conjunctive form of an い-Adjective), followed by て. After this, は is added, and then finally ならない.
ならない is considered to be the most formal structure out of ならない, いけない, and だめ. It is regularly used in formal writing, or semi-formal/formal speech. いけない (the most common variation) and だめ (the most casual variation) are also acceptable in many situations.
Caution

The ては in なくては is often shortened to ちゃ, as this is easier to say. なくちゃ may then be followed by ならない. However, due to ならない being quite formal, なくちゃ (a casual structure) will not be used with ならない on a regular basis. It is more common to see なくちゃ paired with いけない, or だめ.

These casual structures are very similar to words like 'gotta' in English.
N5 文法
なくちゃ・なきゃ
Must do, Have to do
なくちゃ (short for なくては), and なきゃ (short for なければ) are the most common colloquial abbreviations used to convey that something 'must', or 'has to' be done.

Like てはいけない (must not), なくてはいけない (must), and なくてはならない (must), these abbreviation forms can be followed by いけない, ならない, or だめ. However, they are usually omitted, without changing the meaning.

Structures like this are referred to as spoken/colloquial language. These casual variations can come across in a very similar way to words like 'gotta' in English.
In these examples, a variety of forms have been used naturally. There is no 'most natural' form, and it will often depend on the speaker as to which they choose.

As with almost all grammatical structures, the shortest variations (not including いけない, etc.) are the most casual/used the most often.

There are several other ways that these common abbreviations can appear based on region, or even an individual speaker's preference. An example of this is なけりゃ, which is a (very) casual form of なければ.
In this example, we can see that some of these less common variations are used specifically by certain genders (in general). なけりゃ itself tends to be used by male speakers, and may be considered a bit rougher than なきゃ, or なくちゃ, which can be used by anybody.
N5 文法

詳細

This particular group of phrases is initially hard to memorize, as they can be rather long. Let's check the core logic of these phrases and compare them.

No Can Do

For these phrases meaning 'must not', the core logic is always ては (shortened to ちゃ in fast, casual speech) followed by a word which means 'no good'. The literal meaning is similar to 'doing (A) is no good'.

Of the words that follow, ならない is the most formal, objective sounding option. Literally it means 'doesn't become' but is synonymous with 'no good' here. It tends to be used for rules or facts of nature.

いけない is the most neutral option, acting as a fixed phrase meaning 'no good' and used in both speech and writing. It sounds more subjective than ならない does.

だめ is the most casual of these three options, and is even used as a word to scold someone or say something is 'wrong' or 'no good' on its own.

Must I?

For the phrases in this Topic meaning 'must', the core logic is much the same as the 'must not' phrases, but they use the negative form of the verb in question. As mentioned on the grammar pages, this makes a double negative with a literal meaning similar to 'not doing (A) is no good'.

The logic of ては becoming ちゃ and the three options for expressing 'no good' is exactly the same here. One major difference is that we also are able to omit the final 'no good' word and maintain the meaning. This is most common in speech.

We can see some options from most to least formal below.

A Bit Needy

When listed, it can seem like there are a dizzying amount of phrases for 'must' and, to a lesser extent, 'must not'. Unbelievably, there are actually even more than this! We'll learn these in N4, and even in N2 we'll still come across new alternatives.

Although this particular set of phrases can feel intimidating, they are actually not so bad once you are used to them. In all cases, the core logic is exactly the same. We take a negative condition ('If I don't do (A)...') and pair it with a negative result ('...it is unacceptable'). Mastering this double-negative concept now will pay dividends later.

We can see how this logic applies to these N4 phrases below.

Or how the logic applies to this archaic sounding N2 version.

Or how the logic even applies to dialects, like this Kansai-ben (Kobe-Osaka-Kyoto region dialect) phrase.