忘れてはいけない

must not forget

→ 忘れる
phrase Rare #33,571
must not forget Expresses a strong instruction or reminder not to forget something. It can also be translated as “don't forget.”
JA: 大切な人への感謝を忘れてはいけない。
EN: You must not forget to be grateful to the important people in your life.

同じようなことが繰り返されないようにその悲劇忘れてはいけない

We must not forget that tragedy so that something similar does not happen again.

約束を忘れてはいけない

You must not forget your promise.

きみは彼女に多くの借金があるという事実を忘れてはいけない

You mustn't forget the fact that you owe her a lot of money.

君に言ったこと忘れてはいけない

You must not forget what I just told you.