You must look to your health.
健康に注意しなければならない
You must not guess at the answers.
当て推量で答えてはいけない
You must not misbehave so.
そんな無作法をしてはいけない
You must so instruct them that they will not repeat the mistake.
過ちを繰り返さないよう指導しなくてはなりません
You must take his stories with a grain of salt.
彼の話はまともに受け取ってはいけない
You mustn't sign the contract until you hear from me.
私から連絡のあるまで契約書に署名してはいけない
You need to set [put] your mind to your business.
仕事に全力を傾けなくてはいけない
You need to understand how this machine works.
君はこの機械の働きを理解しなければならない
You needn't pay now.
今金を払う必要はない
You needn't write to me unless there is something new.
（あまりないとは思うが）何か新しいことがない限り手紙はよこさなくて結構です（◆You needn't write to me if there isn't anything new. は「何か新...
