You're at a business networking event and meet someone who you might be able to do business with. You want to continue to talk to this person after the event. Near the end of your conversation, you say this.
Let's definitely keep in touch.
Most English learners know "Let's ___", but they often over-use it and say "Let's ___" 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. In the example above, your conversation with the listener has gone well, and you're at a business networking event, so it's natural that you would exchange information.
If you met the person in the example outside of a business networking event, you wouldn't be sure that they wanted to be contacted. So instead of saying "Let's...", you'd say:
I'd like to keep in touch with you.
Or if you want to sound really polite and a little bit shy:
I'd like to keep in touch with you, if you don't mind.
To "keep in touch" means to continue to contact a person. You use this phrase to talk about people that you met in one situation but don't see often. For example, people that you went to school or worked with:
How's Jason doing? Have you guys kept in touch?
People often tell each other "Let's keep in touch", but often don't keep in touch with the people they say this to. And you sometimes use "Let's keep in touch" with people that you don't have any interest in talking to. So when you really want to keep in touch with someone, you have to say it in a stronger way.
Adding the word "definitely" to "Let's keep in touch" helps to show that you really mean it.
You may also need to add more emphasis afterward:
Let's definitely keep in touch. I'm serious.
Let's definitely keep in touch. I mean it, let's talk soon.
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