let someone know idiom
A2 告知，通知
to tell someone something
Let us know when you get there. 你到那儿后告诉我们一声。
Let me know if you need any help. 如果需要帮助就告诉我。
Thank you for coming to the interview - we'll let you know (= tell you whether we are going to offer you a job) in the next week. 谢谢你来参加面试——下周我们会告诉你结果。