昨日 Sakubi: Yesterday's Grammar Guide

Lesson 53: Wrongness with だめ, いけない, ならない, featuring "must", and bonus, some contractions

だめ is a word that means something like "no good", "useless", or "no use". It can be used to say that a statement would be bad, after turning the statement into a noun or a conjunction.

ならない and いけない similar to だめ, but they're strong enough to say that something is prohibited.

ならない and いけない are relatively common, because they're used in a very common double negative construction that means "must".

なければならない

There are several variations on this. They boil down to "not to X" "wrong".

なければならない

なければいけない

ねばならない

ねばならぬ

なかったらいけない

なくてはいけない

etc.

Different ways of forming it have different flavors, levels of strength, and implications about why or in what way it must be done.

ならない is slightly stronger than いけない. ならない has the connotation of mental things that make something undoable, and いけない has the connotation of something being prohibited, but this distinction is in no way objective, and doesn't represent how japanese natives think about it.

On the topic of there being a million ways to say something, there are some contractions that you have to watch out for if you don't have a lot of experience reading. Here are a few. Contractions that were explicitly explained elsewhere in this guide, they aren't listed. Always be careful about contractions in speech and dialogue.