Adam Clarke's Commentary on the Bible

Verse 3 (Psalms 40:3)

A new song - Cheerfulness and joy had long been strangers to him. He seemed to live to utter the most doleful complaints, and be a prey to suffering and wretchedness. Praise for a sense of God's favor was a new song to him. The word is often used to signify excellence: I will sing a most excellent and eminent song.

Many shalt see it - I will publish it abroad and fear-to sin against the Lord, knowing by my example what a grievous and bitter thing it is.

And shall trust in the Lord - Even the worst of sinners shall not despair of mercy, being penitent, when they see that I have found favor in his sight.

- Adam Clarke's Commentary on the Bible