I have more understanding than all my teachers - Referring perhaps to those who had given him instruction in early life. By constant meditation on the law of God, he had, in the progress of years, advanced to a point beyond that to which they had arrived. He had improved upon their suggestions and instructions, until he had surpassed them in knowledge. His "design" in saying this was to set forth the excellency and the fullness of the law of God, and to show how the study of it was suited to enlarge the understanding. In early life the wisdom of teachers seems to be far beyond anything that we can hope to reach; yet a few years of study and meditation may place us far beyond them. What those teachers seemed to be to us, however, when we were young, may serve ever onward as a means of comparison when we wish to speak of the greatness of human attainments. So the psalmist says that he had now reached a point which seemed to him in early life to be wonderful, and to be beyond what he had then hoped ever to attain. He had now reached that point; he had gone beyond it.
For thy testimonies are my meditation - Compare Psalm 1:2; 2-Timothy 3:15. All this knowledge he had obtained by meditation on the law of God; by the study of divine truth. The effect of that constant study was seen in the knowledge which he now possessed, and which seemed to surprise even himself as compared with the brightest anticipations of his early years.

I have more understanding than all my teachers - As he had entered into the spiritual nature of the law of God, and saw into the exceeding breadth of the commandment, he soon became wiser than any of the priests or even prophets who instructed him.

I have more (b) understanding than all my teachers: for thy testimonies [are] my meditation.
(b) Whoever submits himself only to God's word will not only be safe against the practises of his enemies, but also learn more wisdom than they who profess it, and are men of experience.

I have more understanding than all my teachers,.... Such as had been or would have been his teachers, who were bad ones in religious matters; especially such might be the religious teachers in Saul's time, when David was a young man: as the priests, whose lips should keep knowledge, and deliver it to the people, were in the times of Malachi; and as the Scribes and Pharisees, who, sat in Moses's chair, were in Christ's time; and as those legal teachers were in the apostles' times, who would be teachers of the law, not knowing what they said, nor whereof they affirmed; such as these David exceeded in spiritual understanding. Or his good teachers are meant; and though in common it is true that "a disciple is not above his master", Matthew 10:24; yet there are sometimes instances in which scholars exceed their teachers in knowledge and learning; and this is no reproach to a master to have such scholars: no doubt Apollos so improved in knowledge as to excel Aquila and Priscilla, of whom he learned much; as the Apostle Paul excelled Ananias; and so David excelled his teachers: and which is said by him, not in an ostentatious way of himself, nor in contempt of his teachers; but to commend the word of God, the source of his knowledge; and to magnify the grace of God, to whom he attributes all his wisdom, as in Psalm 119:98. Kimchi interprets it,
"of them all I have learned and received instruction; and from them I have understood the good way, and they have taught me;''
for thy testimonies are my meditation; what he learned of his teachers he compared with the word, the Scriptures, which testify of the mind and will of God; he searched into them, he meditated upon them, and considered whether what his instructors taught him were agreeable to them or and by this means he got more understanding than they had.

understanding--is practical skill (Psalm 2:10; Psalm 32:8).