Blessed is the man unto whom the LORD imputeth not iniquity, and in whose spirit there is no guile.
Imputeth — Whom God doth not charge with the guilt of his sins, but graciously pardons and accepts him in Christ.
No guile — Who freely confesses all his sins, and turns from sin to God with all his heart.
 When I kept silence, my bones waxed old through my roaring all the day long.
Silence — From a full and open confession of my sins.
Old — My spirit failed, and the strength of my body decayed.
Roaring — Because of the continual horrors of my conscience, and sense of God's wrath.
 For day and night thy hand was heavy upon me: my moisture is turned into the drought of summer. /*Selah*/.
Hand — Thy afflicting hand.
My moisture — Was dried up.
 I acknowledged my sin unto thee, and mine iniquity have I not hid. I said, I will confess my transgressions unto the LORD; and thou forgavest the iniquity of my sin. /*Selah*/.
The iniquity — The guilt of my sin.
 For this shall every one that is godly pray unto thee in a time when thou mayest be found: surely in the floods of great waters they shall not come nigh unto him.
For this — Upon the encouragement of my example.
Found — In an acceptable and seasonable time, while God continues to offer grace and mercy.
Waters — In the time of great calamities.
Not come — So as to overwhelm him.
 I will instruct thee and teach thee in the way which thou shalt go: I will guide thee with mine eye.
I will — This and the next verse seems to be the words of God, whom David brings in as returning this answer to his prayers.
Mine eye — So Christ did St. Peter, when he turned and looked upon him.
 Be ye not as the horse, or as the mule, which have no understanding: whose mouth must be held in with bit and bridle, lest they come near unto thee.
Will not — Unless they be forced to it by a bit or bridle. And so all the ancient translators understand it.
 Many sorrows shall be to the wicked: but he that trusteth in the LORD, mercy shall compass him about.
Sorrows — This is an argument to enforce the foregoing admonition.