To say that you have to or must do something, you can use a verb in plain negative form + ~と　いけない. Here the particle と in this phrase can be translated as if, so the phrase literally means, if not ... then it can't go.

You can also use いけません in place of いけない to make the sentence more formal.

It's also possible to replace といけない with とダメです. ダメ essentially means no good, so the phrase verb in plain negative form + ~と　ダメです literally means if not ... then it's no good. Note that ダメ is spelled using katakana here, but in other cases it's often spelled using hiragana.

In very informal situations you can simply use the verb in plain negative form + ~と, omitting the final いけない.

There are also different ways to say must do, but we will look at those in a later lesson.

Breakdown of kodomo ha mainiti gakkou ni ikanai to dame desu.