Sing unto the Lord with thanksgiving - Accompany the praise of God - the expression of worship - with a grateful remembrance of the past. The one will aid the other, and the two will constitute acceptable and proper worship. The first word here means properly to answer, or respond; and the idea would seem to be, that we are to make a suitable response or answer to the manifold layouts which we have received at the hand of God.
Sing praise upon the harp unto our God - On the word harp, see the notes at Isaiah 5:12. The harp was an instrument commonly employed in divine worship. See the notes at Psalms 33:2 : “Praise the Lord with harp.” Compare Psalms 43:4; Psalms 49:4; Psalms 57:8; Psalms 71:22.