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.
I’m so tired already. I have to sleep soon. | もう、 とても 疲れて いる。 すぐ 寝ない と いけない。mou, totemo tsukarete iru. sugu nenai to ikenai. |
That teacher’s class is hard. The homework has to be written in Japanese. | あの 先生 の 授業 は 難しい です。 宿題 は 日本語 で 書かない と いけない。ano sensei no jugyou wa muzukashii desu. shukudai wa nihongo de kakanai to ikenai. |
You can also use いけません in place of いけない to make the sentence more formal.
We have to clean before the guests come. | お客さん が 来る 前 に、 掃除しない と いけません。okyakusan ga kuru mae ni, soujishinai to ikemasen. |
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.
Children have to go to school every day. | 子供 は 毎日 学校 に 行かない と ダメ です。kodomo wa mainichi gakkou ni ikanai to dame desu. |
In very informal situations you can simply use the verb in plain negative form + ~と, omitting the final いけない.
Watch out. It’s super hot. You have to drink some water. | 気 を 付けて ね。 めちゃ 熱い。 水 を 飲まない と。ki o tsukete ne. mecha atsui. mizu o nomanai to. |
There are also different ways to say must do, but we will look at those in a later lesson.