But his delight is in the law of the LORD; and in his law doth he meditate day and night.
But - Hebrew, kiy (H3588) 'im (H518), but if; i:e., But who (if there be anything in which he) delights (Romans 7:22), (it is) in the law ( towraah (H8451), literally, the directory, from a root, yaarah (H3384), to point the way or direct; the law of Moses: the representative of the whole inspired volume) of the Lord. So Maurer. Thus even the Psalms are called "the law" of the Jews (John 10:34; John 15:25). Jesus, the faultless model-man, pre-eminently could say, "I delight to do thy will: thy law is written within my heart" (Psalms 40:8). The renewed and believing man delights in God's law sincerely, though in a lower degree (Romans 7:22; cf. Psalms 119:16; Psalms 119:35; Psalms 119:47).
The Lord - Hebrew, Yahweh (H3068). So the English version is to be understood wherever the LORD is in capital letters. Compare as to the meaning of the name, Exodus 3:14-15 ("I am that I am;" or, 'I shall be (always) that I am (now),' according to Gesenius), and Revelation 1:4; Revelation 1:8, notes.
Meditate day and night - alluding to Joshua 1:8. Meditation upon, is to reading the Word what digesting is to eating. Without the slow and lengthened process of digestion, food would not nourish the body: without meditation, the Word read will not nourish the soul.