Concerning thy testimonies, I have known of old that thou hast founded them for ever.

I have known from the beginning concerning thy testimonies: that thou hast founded them for ever.

Of old I have known Thy testimonies, That to the age Thou hast founded them!

I have long had knowledge that your unchanging word is for ever.

Of old have I known from Thy testimonies that Thou hast founded them for ever.

Of old I have known from your testimonies, that you have founded them forever.

I have known from thy testimonies. from the beginning. Others here translate, I have known long ago of thy testimonies. [1] This translation I would not directly reject; but I am more inclined to retain the sense which I have given, namely, That the Prophet not only knew the everlasting steadfastness which characterizes the testimonies of God; but that he had also derived this knowledge from the testimonies themselves. When the Hebrews would express the meaning conveyed by the Latin preposition de, they frequently use the particle mn, or the letter v, beth. He therefore says, that he had learned from God's testimonies, or had been taught by them, that they are established for ever [2] This indeed is the chief point of faith, That the word of God is not only distinguished for fidelity and steadfastness for a time, but that it continues unchangeable for ever. Were it otherwise, it could not include within it the hope of eternal salvation. That the assurance of this immutabiliy of God's word may be rooted in our minds, the inward revelation of the Holy Spirit is indeed necessary; for until God seal within us the certainty of his word, our belief of its certainty will be continually wavering. Yet the Prophet, not without cause, affirms, that he learned this truth from the word; for when God shines into us by his Spirit, he at the same time causes that sacred truth which endures for ever to shine forth in the mirror of his word.

Footnotes

1 - Walford's rendering is, "I have known thy testimonies long since." Phillips translates "of old;" and gives this explanation, "I have been acquainted with thy testimonies ever since I have possessed any knowledge, i.e. as soon as I came to years of reflection. From a child thou hast known the holy scriptures.' 2-Timothy 3:15."

2 - Thou hast established them for ever. That is, thy revelations are unalterable and everlasting, as the attributes of their great Author, and can never fail those who rely upon them, in time, or in eternity." -- Warner on the Psalter.

Concerning thy testimonies - In regard to all that thou hast testified to as true and best. Every command of God is in fact a testimony of his as to what is right; every promise is a testimony of his own purpose in regard to mankind.
I have known of old - The word used here is a noun, and means properly, "the front," what is "before;" then, the East; then, what pertains to olden time or ancient days - "before" the present. The meaning here is, that he had known this "before" what had now occurred; it was not a new thing - a new experience. It was deeply impressed on his mind as the result of all his reflection and observation.
That thou hast founded them for ever - "From" eternity, and "for" eternity. They were laid in the eternity past; they will continue in the eternity to come. They are based on eternal principles of right; they will never be changed. Such a conviction will do much to keep the soul steady and firm in the trials and uncertainties of life. Whatever may change, God's law does not change; whatever is new, that is not new; whatever will vanish away, that will remain.

Concerning thy testimonies, I have known of old - קדם ידעתי kedem yedati, "Long ago I have known concerning thy testimonies." Thou hast designed that thy testimonies should bear reference to, and evidence of, those glorious things which thou hast provided for the salvation of men; and that this should be an everlasting testimony. They continue, and Christ is come.

(d) Concerning thy testimonies, I have known of old that thou hast founded them for ever.
(d) His faith is grounded on God's word, that he would always be at hand when his children are oppressed.

Concerning thy testimonies, I have known of old,.... Or, "from thy testimonies, I have known of old" (d): by carefully reading the Scriptures which testify of God, his mind an will, and frequently meditating on them, he had learned a long time ago, even from his youth, what follows,
that thou hast founded them for ever; that the things contained in them are sure and certain, established and eternal truths; the moral law and the precepts of it are eternal, and of perpetual obligation; not one jot or tittle of them shall ever fail; the Gospel, and the truths of it, are everlasting, and shall ever remain; in spite of all the opposition, craft and cunning, fury and force of men, to undermine and root them out; see Psalm 119:89.
(d) "ex obtestationibus tuis", Tigurine version; so Cocceius, Gejerus.

Of old - By long experience. Founded - Thou hast established them upon everlasting foundations.