What is the difference between When the knock came at the door, I was practicing guitar,... and I've been learning the guitar.?
Feel free to just provide example sentences.

Oh, didn’t notice you’re asking for the difference. I misread thought you’re which one is more natural.

When the knock came at the door, I was practicing guitar- this was happening at a specific moment in the past or two ongoing actions were happening at the same time.

Ex: You were practicing guitar when she called.

I've been learning the guitar.-happened in the past and continues to the present. So it means you started to learn in the past and still learning the guitar now or until recently.

Ex: I’ve been studying English(you started before and still studying now)


*usually for this situation we use when/while plus verb-ing .
Ex: When the phone rang, she was writing a letter.