This psalm also has no title to indicate the author, or to explain the occasion on which it was composed. It is a psalm of very special construction, and stands alone in the form of its poetry. The peculiarity consists in repeating at the close of each verse the language "for his mercy endureth forever." This is a kind of refrains, and may have been designed, in public worship, to be a response by a choir, or by the people. That it may have been intended to be so used cannot be disproved, nor can anyone show that such a response in public worship is, itself, improper or wrong. It is not certain, however, that it was meant to be so used; and it should not, therefore, be appealed to as proving that such responses are proper in public worship, whatever may be true on that point. It may have been merely a specimen of the poetic art among the Hebrews - one of the forms in which Hebrew poetry expressed itself. The subjects referred to as laying the foundation for the response in each verse - "for his mercy endureth forever," are such as have been often introduced in the previous psalms, and will require but little additional illustration. The general idea is, that all these acts of the divine interposition - all that God has done, even though it seemed to be a display of power or of justice, of severity or of wrath - was, in fact, an illustration of the "mercy" of God, and laid a foundation for praise. That is, All this was connected with the good of his people, with favors to mankind, with the accomplishment of great and benevolent purposes, and, therefore, was expressive of mercy - a proof that the "mercy of God endures forever."

An exhortation to give thanks to God for various mercies granted to all men, Psalm 136:1-9; particularly to the Israelites in Egypt, Psalm 136:10-12; at the Red Sea, Psalm 136:13-15; in the wilderness, Psalm 136:16-20; and in the promised land, Psalm 136:21, Psalm 136:22; for the redemption of the captives from Babylon Psalm 136:23, Psalm 136:24; and for his providential mercies to all, Psalm 136:25, Psalm 136:26.
This Psalm is little else than a repetition of the preceding, with the burden, כי לעולם חסדו ki leolam chasdo, "because his mercy endureth for ever," at the end of every verse. See below. It seems to have been a responsive song: the first part of the verse sung by the Levites, the burden by the people. It has no title in the Hebrew, nor in any of the Versions. It was doubtless written after the captivity. The author is unknown.

INTRODUCTION TO PSALM 136
This psalm was very probably composed by David, and given to the Levites to sing every day, 1-Chronicles 16:41. Solomon his son followed his example, and made use of them in singing at the dedication of the temple, 2-Chronicles 7:3; as Jehoshaphat seems to have done when he went out to war against his enemies, 2-Chronicles 20:21. The subject of it is much the same with the preceding psalm; its composition is very singular; the half of every verse: in it is, "for his mercy endureth for ever"; this is the burden of the song; and the design of it is to show, that all blessings of every kind flow from the grace, goodness, and mercy of God, which is constant and perpetual; and to impress a sense of it upon the minds of men: the inscription of the Syriac version is,
"it is said of Moses and Israel praising the Lord for those who were delivered; and concerning the deliverance of souls out of hell from Pharaoh, the devil, by Christ our Saviour, the Redeemer of them.''
R. Obadiah says it is an exhortation to the children of God in the days of the Messiah to praise the Lord.

(Psalm 136:1-9) God to be praised as the Creator of the world.
(Psalm 136:10-22) As Israel's God and Saviour.
(Psalm 136:23-26) For his blessings to all.

O Give Thanks unto the Lord, for He Is Good
The cry Psalm 135:3, Praise ye Jāh, for good is Jahve, is here followed by a Hodu, the last of the collection, with "for His goodness endureth for ever" repeated twenty-six times as a versus intercalaris. In the liturgical language this Psalm is called par excellence the great Hallel, for according to its broadest compass the great Hallel comprehends Ps 120-136,
(Note: There are three opinions in the Talmud and Midrash concerning the compass of the "Great Hallel," viz., (1) Ps 136, (2) Ps. 135:4-136:26, (3) Ps 120-136.)
whilst the Hallel which is absolutely so called extends from Psalm 113:1-9 to Ps 118. Down to Psalm 136:18 the song and counter-song organize themselves into hexastichic groups or strophes, which, however, from Psalm 136:19 (and therefore from the point where the dependence on Ps 135, already begun with Psalm 136:17, becomes a borrowing, onwards) pass over into octastichs. In Heidenheim's Psalter the Psalm appears (after Norzi) in two columns (like Deut. 32), which it is true has neither tradition (vid., Ps 18) nor MSS precedent in its favour, but really corresponds to its structure.