Why not "Ki ismeri a híres ugyvedet"
That's good, too - with the right accents for the lawyer.
Why is the plural not accepted, who said who refers to singular?
Probably not in the database yet.
can we say A híres ügyvédet ki tudja?
No, tud is for knowing a fact, or knowing how to do a thing ("can" or "be able to" do something). Ismer is for "being acquainted with" people (or places or other things).
Ha híres, mindenki ismeri!
why is there an i at the end? ismerni: ismerek...
That's definite conjugation, because the direct object in this sentence, the lawyer, is a definite object, marked with a. A single certain lawyer. Definite conjugation uses a separate set of conjugational suffixes.
thank you very much