Japanese Lesson: Must or must not do something
In this lesson, you'll learn how to say things like "I must do my homework", "my little sister must not lie", etc. I'll be teaching the formal way to do express it, if anyone really wishes I can explain the 'shortcuts' or the informal version.
Must Not Do
First, we’ll talk about how to say that you must not do something
To conjugate this, you have to conjugate to the -te form of the verb (if you don't know how to do this, don't worry; a lesson on it will be made later). After it's conjugated to the -te form, you than add は followed by either だめ、いけない、Or ならない.
The differences between these three are pretty simple. だめ is a word that means 'bad' or 'no good', and there is not much of a difference between ならない And いけない, other than ならない is normally used for things like set rules that apply to more than one person.
To say something like "You can't eat that", you would use the verb 食べる "to eat", conjugate it to 食べて, and then add はだめ、はいけない、はならない
それを食べてはだめ- You can't eat that (it's bad if you do)
それを食べてはいけない- You can't eat that
それを食べてはならない- You can't eat that (it's a rule that you can't eat that)
車（くるま）に乗（の）ってはならない- You can't ride in the car
そんなことを言（い）ってはダメ- You can't say something like that
Negative te-form (will be covered in the te-form lesson)+は+だめ、いけない、ならない
毎日（まいにち）学校に行かなくてはなりません- Must go to school every day
それを食べなくてはいけない- Have to eat that
I apologize if that was a little hard to follow, and I know that it has some grammar points that people may not know. If you have any questions, you can't comment and I can try to answer.
You're never going to hear それはダメ unless you're either a really young child or it's obvious nothing else will work. Even then, adults will probably use "no" if you look like you can understand English before 何々はダメ.