Two verses, one sentence. Verse 1 is a double imperative addressed to the nations, verse 2 is the reason, introduced by the particle for. The parallelism is exact: praise matches laud, גּוֹיִם (go-yim, v.1) matches הָאֻמִּים (ha-u-mim, v.1), and the two lines of verse 2 pair kindness with faithfulness, a standard word pair across the psalms. The closing Halleluyah is not part of the sentence; it is the sign that this psalm belongs to the Hallel group, and the text here writes it as two words, an imperative plus the short divine name at יָהּ (yah, v.2). Nothing here is narrated, promised or requested. The psalm asks for one action and gives one ground for it.
The verb at גָבַר (ga-var, v.2) is the crux. Its root is the one behind strength and prevailing, used elsewhere of a river overwhelming a bank or a force overcoming an opponent, so the literal sense is that His kindness has overpowered us. Translations soften it to great toward us or abundant toward us, which loses the violence of the image: kindness here is something that wins, not something that is offered. The two nouns in verse 1 are also not synonyms by default. The first, גּוֹיִם (go-yim, v.1), is the ordinary word for the nations of the world, while the second, הָאֻמִּים (ha-u-mim, v.1), is rarer and stands for peoples in the sense of populations, so the pair widens the address rather than repeating it. The paired terms in verse 2, kindness and וֶאֱמֶת (ve-e-met, v.2), are the same two that appear together as a formula elsewhere in the Torah and the psalms.
The obvious question is why the nations should praise for a kindness that prevailed over us rather than over them, and the readings split. One takes the nations as witnesses: they saw what was done for Israel from outside, and testimony from outside is worth more than self report, which is why they and not we are commanded here. Another reads over us as against us, that is, God's commitment held even when we gave Him reason to drop it, and the nations who watched that are the ones who can vouch for it. A third turns it around and takes the nations as beneficiaries too, since what was done for one people set the terms for everyone. All three are ancient readings and the text supports each of them; picking one silently would be a translation choice pretending to be a fact.
In the mystical reading the two verses are a hinge rather than a poem. Kindness that prevails is an overflow that cannot be contained by the vessel it was given to, so it spills outward, and the address to the nations is that overflow arriving at its far edge: praise reaching the place that was not the recipient. The shortness is read as part of the meaning rather than an accident of transmission, since a psalm with nothing in it but an imperative and a reason leaves no room for a self, and the final short name at יָהּ (yah, v.2) is treated as the contracted form of the divine name, the one used when the fullness is deliberately held back.