Behold, thou desirest truth in the inward parts: and in the hidden part thou shalt make me to know wisdom.
Behold. The repetition of exclamation (cf. Psalms 51:5) implies the close connection of Psalms 51:5-6 -`My very nature, as that of all mankind, is tainted with sin, but thou desirest sincere righteousness: therefore there urgent need that thou, who alone canst, shouldst impart righteousness, and its first beginning, the pardon of sin.'
Thou desirest truth in the inward parts - "truth," i:e., true righteousness, as opposed to self-deceit and hypocrisy, of which he was guilty all the time (nearly a year) that he continued living in sin, though not giving up the outward profession of a worshipper of God (Joshua 24:14; 1 Kings 2:4; John 3:21).
And in the hidden part - the hidden depths of the heart, which, in the natural man, however outwardly wise, are replete with folly. Compare the contrast of "inwardly" and "outwardly," Romans 2:28-29.
Thou shalt make me to know wisdom - true Wis 1:1-16 :e., piety, internally initiated by the Holy Spirit, so as to act on my whole spirit and life (cf. Job 38:36). Virtually a prayer in the form of a confident anticipation. As thou desirest true righteousness, and I neither have it naturally nor can acquire it of myself, 'thou shalt give' it, I trust and pray. So all the succeeding future tenses are optative (cf. Psalms 51:10; Psalms 51:12).