Yahweh's testimony is sure, making wise the simple. The testimony is this, that God gave to us eternal life, and this life is in his Son. He who has the Son has the life. He who doesn't have God's Son doesn't have the life. These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God.
Precepts are the rules of life.
The Sermon on the Mount, Matthew chapters 5 through 7, gives us a number of good examples of these rules. Everyone therefore who hears these words of mine, and does them, I will liken him to a wise man, who built his house on a rock. Everyone who hears these words of mine, and doesn't do them will be like a foolish man, who built his house on the sand. Yahweh's commandment is pure, enlightening the eyes. This is his commandment, that we should believe in the name of his Son, Jesus Christ, and love one another, even as he commanded.