OET interlinear PSA 89:37

uW Translation Notes:

Note 1 topic: figures-of-speech / simile

כְּ֭⁠יָרֵחַ יִכּ֣וֹן עוֹלָ֑ם

(Some words not found in UHB: his/its=seed forever,perpetuity will_belong and,throne_of,his like,sun before,me )

The point of this comparison is that just as the moon is permanent and enduring, so is the dynasty of David. If it would be helpful in your language, you could make this point explicitly. Alternate translation: [I will establish it forever, as permanent as the moon]

Note 2 topic: figures-of-speech / activepassive

יִכּ֣וֹן

(Some words not found in UHB: his/its=seed forever,perpetuity will_belong and,throne_of,his like,sun before,me )

The expression It will be established is a passive verbal form. If it would be unnatural to use that form, you could express the idea in active form. If you need to say who does the action, the context indicates that it is God. Alternate translation: [I will establish it]

Note 3 topic: figures-of-speech / metaphor

וְ⁠עֵ֥ד בַּ֝⁠שַּׁ֗חַק נֶאֱמָ֥ן

(Some words not found in UHB: his/its=seed forever,perpetuity will_belong and,throne_of,his like,sun before,me )

Here God is speaking of the moon as if it were the attested witness in the sky, as though it testifies to the permanence of his covenant. Your language may have a comparable expression that you can use in your translation, or you could state the meaning plainly. Alternate translation: [which is a faithful sign in the sky]