Deal with thy servant according to thy goodness. The two clauses of this verse must be read correctly; for he does not first separately desire God to deal well with him, and next desire him to be his master and teacher. He rather beseeches him in the exercise of that goodness and mercy, which he is wont to display towards all his people, to instruct him in his law. The object of the Prophet's request then is, that God would teach him in his statutes. But he begins with the divine mercy, employing it as an argument to prevail with God to grant him what he desires. This prayer then must be resolved thus: Lord, deal gently with me, and manifest thy goodness towards me by instructing me in thy commandments. Our whole happiness undoubtedly consists in our having that true wisdom which is to be derived from the word of God; and our only hope of obtaining this wisdom lies in God's being pleased to display his mercy and goodness towards us. The Prophet, therefore, magnifies the greatness and excellence of the benefit of being instructed in the divine law, when he requests that it may be bestowed upon him as a free gift.

Deal with thy servant according unto thy mercy - Not according to justice - for, sinners as we are, we can never urge that as a plea before God. No man who knows himself could ask of God to deal with him according to the strict and stern principles of justice. But we may ask him to deal with us according to mercy - for mercy is our only plea, and the mercy of God - vast and boundless - constitutes such a ground of appeal as we need. No man can have any other; no man need desire any other.
And teach me thy statutes - See the notes at Psalm 119:12. Show thy mercy to me in teaching me thy law.

Deal with thy (b) servant according unto thy mercy, and teach me thy statutes.
(b) He does not boast that he is God's servant, but by this reminds God that as he made him his by his grace, so he would continue his favour toward him.

Deal with thy servant according unto thy mercy,.... Which is either general and providential, and reaches to all his creatures; and according to which David had been dealt with all his days, and which he desires a continuance of: or special; and which is in Christ, and communicated through him; and in whom he deals with his people, not according to their merits, but his own mercy; by receiving and accepting them, and admitting them into his presence, and to partake of his favours, and by pardoning their sins and saving their souls; which is not by works of righteousness they have done but according to his abundant mercy; and by giving them eternal life and happiness at the great day;
and teach me thy statutes; which is often requested; and which not only shows the need of divine teachings, and the psalmist's earnest and importunate desire to have them; but also that the mercy, grace, and kindness of God, have an influence on the holy life and conversation of the saints, and do not at all encourage licentiousness.