I understand what you are tellingme but I can't speak.
I understand what you are speaking about but I can't speak.
I understand what you are talking about but I can't speak.
I understand what you are saying but I can't speak.
tell - говорить (рассказывать)
say - сказать (что-то)
speak - говорить (разговаривать, выступать)
talk - говорить (разговаривать)