Let me know is a request to tell you about a decision, news or a problem once there is something to tell. It sounds neutral and polite, so it suits both conversations with friends and work emails. Let me know if you have any questions is one of the most common ways to end a business email.
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let me know + if/when/what/whether + clause; let me know about + noun; let + person + know; I'll let you know
- Let me know when you land, and I'll come and pick you up.
- If you have any questions about the report, just let me know.
- Could you let me know by Friday whether you can join the trip?
What changed
First versionPlease let me to know if you need any help with the move.
RevisionPlease let me know if you need any help with the move.
After let, the verb comes without to: let me know, let him go, let us help. To slips in by analogy with patterns like ask me to call or want me to know, where it is needed.
Tell me sounds more direct and asks for an answer now: Tell me what happened. Let me know is softer and leaves time to find something out or decide: Let me know how the interview goes, meaning “tell me later how it went”.
Your turn
I haven't decided about the trip yet, but I'll ___ by Friday.
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let you know — The speaker is the one deciding, and they promise to tell the other person later, so I'll let you know. Let me know asks someone else to tell you, so after I'll it makes no sense.
You've sent a colleague a draft of a report. Write them a message: use let me know to ask what they think, and give a deadline.
I've sent you the draft of the report, so please let me know what you think by Wednesday.
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