A question about "δὲ"

Hello. I recently started learning Greek using the Reading Greek textbooks published by Cambridge.
I have a question about the word “δὲ”. Its translation is given as “and; but.”
I am not clear what is its purpose. For example,
“ἐγὼ δὲ οὐκ ακούω, ὦ φίλοι.”,
should be translated, I think, as
“I do not hear, friends.”
If that is so, why the Greek sentence isn’t
“ἐγὼ οὐκ ακούω, ὦ φίλοι.”?
Thank you.

Well, like “and” and “but”, δέ can only be understood in relation to something that precedes. So your in your example

ἐγὼ δὲ οὐκ ακούω, ὦ φίλοι.

δέ means that the speaker is reacting to something that precedes, probably to something someone else has said.

But I can’t/don’t hear, friends.”

In the same way, “but” can only be understood if we know the situation where the speaker is.