The KJV is much more literal here than the NASB.
(Proverbs 21:28 NAS77) A false witness will perish, But the man who listens to the truth will speak forever.Notice the words not in italics. They are not in the Hebrew.
The second part of this verse reminds us of an important principle in the life of the righteous, the importance of listening. Too often we tend to spout off. I know that I often will start answering before the question has been asked and that often leaves me hanging out like a fool.
Much of what we say and do will be forgotten. Make that most. What strikes me as I teach is that students who don’t seem to be listening will perk up when I stick my foot in my mouth. They seem to have radar that wakes up when a stray word wanders into their daydreams. And that is what they remember.
So? Listen. Take time to hear. Then open your mouth and share the benefits of your listening.