请问知已知彼用英文怎么说？

Know yourself, know your enemy 了解自己，了解你的敌人

英语是没有成语得，所谓成语是语言中经过长期使用、锤炼而形成的固定短语。它是比词大而语法功能又相当于词的语言单位。

have the knowledge of oneself and the other side

estimate correctly one's own strength as well as that of one's opponent