Adam Clarke's Commentary on the Bible

Verse 9 (Psalms 19:9)

The fear of the Lord - יראה yirah , from ירא yara , to fear, to venerate; often put for the whole of Divine worship. The reverence we owe to the Supreme Being.

Is clean - טהורה tehorah , from טהר tahar , to be pure, clean; not differing much from ברה barah , (see above), to be clean and bright as the heavens; as purified Silver. Its object is to purge away all defilement, to make a spotless character.

Enduring for ever - לעד עומדת omedeth laad , standing up to Perpetuity. The fear that prevents us from offending God, that causes us to reverence him, and is the beginning as it is the safeguard of wisdom, must be carried all through life. No soul is safe for a moment without it. It prevents departure from God, and keeps that clean which God has purified. This is Its use.

The judgments of the Lord - משפטים mishpatim , from שפת shaphat , he judged, regulated, disposed, All God's regulations, all his decisions; what he has pronounced to be right and proper.

Are true - אמת emeth , truth, from אם am, to support, confirm, make stable, and certain. This is the character of God's judgments. They shall all stand. All dispensations in providence and grace confirm them; they are certain, and have a fixed character.

And righteous altogether - They are not only according to truth; but they are righteous, צדקו tsadeku , they give to all their due. They show what belongs to God, to man, and to ourselves. And hence the word altogether, יחדו yachdav , equally, is added; or truth and righteousness united.

- Adam Clarke's Commentary on the Bible