Your friend is learning to play the guitar. He's frustrated because he doesn't think he's improving. But you do think he's improving, you say this to encourage him.
I can definitely tell you're getting better.
When you "can tell" something, it means that you know it based on something you saw:
I can tell that you're lying to me.
Could you tell I wanted to leave?
You can leave "that" out in casual conversation, but in formal speaking or in writing you should definitely include it:
I can definitely tell that you're improving.
Use "___ can tell ___" to talk about things that someone noticed out on their own. Don't use if for things that someone heard from other people. In that situation use a phrase like "I heard ___":
I heard you and Desmond weren't getting along.
When you use "definitely", it means that you're sure:
I'm definitely taller than him. He's like two inches shorter than me.
"Definitely" is often used together with "can tell":
I can definitely tell that you've lost some weight.
When you're "getting better", it means you're improving:
I'm getting better, slowly but surely.
"Get better" also means to recover after a sickness or injury, by the way.
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