Must and must not

Motivation

This is the grammar point I most had in mind when starting the site.

When grinding a grammar srs a while ago it felt like the entry with the most different variations, without much reasoning as to when you should use one or another. I don't think this post will even cover all the different ways you can say you must or must not do something (i'm sure it'll be added to later) and I am fully aware I have walked into a minefield by starting with grammar so linked to politeness, so treat it like a light introduction to the incoming dread of knowing that the language you are learning (much like the language you already know) has a bunch of wack rules and different ways to get your point across and you'll have to learn as many as possible.

Terrifying!

{tf}

Must not

「verb」+ てはいけない - must not, may not

Most basic to start.

The ては is a combination of particles (guess which ones!).

The て particle links clauses with a vague notion of chronological or causal event order, and the は particle is the classic subject marker. Together they indicate that the following statement comes as a result of the previous clause.

i.e. the clause following ては comes as a result of the 「verb」before it.

いけない (行けない) is the negative potential form of 行く "to go", altogether coming to literally mean "as a result of 「verb」 you cannot go".

The いけない portion can be replaced with the more formal いけません, and just to get extra fun ならない "cannot become" 1 and だめ "no good".

  • ならない seems to refer more to societal duty (i.e. You shouldn't commit crimes) and is used in legal documents and is the most formal of the three.

  • いけない seems to refer more to personal duty (i.e. I shouldn't eat so much) and is very slightly more colloquial as well as a lot more common in speech, but can mostly be used interchangeably with ならない.

  • だめ is a lot more casual compared to the other two, and is more often used to project warning or criticism (i.e. You can't watch TV, You can't give up).

Must

「verb」+ なくてはいけない - must, have to

Similar to the "Must not" grammar point, this instead turns into a double negative with the inclusion of なくて.

Conjugating a ない form verb to なくて 2 essentially makes the entire structure "as a result of not doing「verb」you cannot go".

As with てはいけない for "must not", いけない can be replaced with ならない and だめ, carrying the same levels of politeness and nuance.

「verb」+ なければいけない - must, have to

Continuing the trend of "must do" being a double negative, in comes なければいけない.

Eschewing from the previously used 「ない conjugate」 + 「ては」form, this structure uses the ば particle with a verb conjugated in negative conditional form 3 (it's not that bad).

As far as I can tell this does not have an effect on how the phrase comes across, being much more affected by the connotations from (you guessed it) also being able to use ならない and だめ!

なくちゃ・なきゃ - colloquialisms

As the above structures are quite wordy, they can both be shortened for use in casual conversation to achieve a meaning similar to "gotta" or "hafta".

なくては becomes なくちゃ, and なければ becomes なきゃ.

いけない, ならない, and だめ can follow both, but is usually omitted without changing the meaning.

Both sound a little cutesy, but I have seen wildly varying reports from "no one over the age of 6 would use them" to "I have seen many manly men use them frequently" so take that as you will.

「verb」+ ないと - must, have to

For our final structure, another double negative with the form ないと.

This uses the "if" meaning of the と particle 4 to express the same thing as ば and ては in the above structures.

Unlike the above, ならない is generally not used with ないと. However, いけない and だめ are still used, as well as ないと with nothing following it.

As is has the 'and' nuance (see the other uses of the と particle 5) it has more strength than なければ or なくては resulting in it often being used to highlight things that absolutely must be done (following the law, etc).

Overall

Most ways to say "you must" are a negative verb, followed by some sort of conditional structure, followed by either ならない, いけない or だめ to form a double negative.

If you don't 「verb」, (lit. it will not become/it will not go/not good).

You say "you must not" by using a regular verb, followed by the ては conditional structure, followed by one of the same three endings

If you 「verb」, (lit. it will not become/it will not go/not good).

In general, from most to least formal (a very rough guide):

  • なりません

  • ならない

  • いけません

  • いけない

  • だめ

  • なくちゃ・なきゃ

From strongest to weakest:

  • ないと

  • なければ・なくては

  • なくちゃ・なきゃ

Further reading:

  1. See 成る on jisho. Can also be conjugated to a more formal なりません anywhere else it's used in this article.↩
  2. I haven't really explained this, if you are interested about the conjugation of negative verbs see this bunpro page for more info↩
  3. see 〜ば conditional form on Tofugu↩
  4. see 〜と conditional form on Tofugu↩
  5. see the と particle on Tofugu↩