“Let's... talk about that offline.”
You're leading a large meeting at work. One of the attendees in the meeting asked you a question. The answer is only important for that person, so you don't want to take up everyone else's time. You respond this way.
Let's... talk about that offline.
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Let's (do something).
The phrase "Let's ___" is well-known to English learners. However, learners often over-use "Let's ___" and use it in situations where it isn't appropriate.
When can you say "Let's ___"? You usually use it when you're suggesting something that you're sure the listener will accept. If you're meeting a friend for lunch, then you're sure that they will agree to eat with you. So you can say:
In situations where you're not sure how the listener will respond to your suggestion, you can use other phrases like "Would you like to ___?", "Why don't we ___?" or "We should ___":
Would you like to go out to dinner with me some time?
Why don't we meet at 7:30?
We should get together some time and have a drink.
talk about (something) offline
When you discuss something in a meeting, everyone can hear what you're talking about. If you don't want everyone to hear your discussion, you can suggest talking about it "offline":
I don't want to get into that here, but we can talk about that offline.
The word "offline" comes from computers. When something is "offline" it's not connected to the Internet or to another system.
You can use "talk about ___ offline" in situations like Internet forums or email groups. Two people can discuss a topic "offline", which means discussing it directly on email or through direct messages.