In the third verse of "Dragonborn" there's a line that goes "Ahrk fin kel lost prodah." The use of "fin" there seems odd to me, because the Elder Scrolls are supposed to be important. Shouldn't they have used "faal," like when they referred to "faal krein" in the previous line? Is this an exception to the usual rules? Or do the dov have no reverence for the Elder Scrolls, as they are powerful and (mostly) immortal themselves?

That's a good question! This could be an oversight by the writers, of course, but you bring up an interesting point. There is no reference to the Elder Scrolls being mentioned with "faal" attached at any point in the game. Paarthurnax speaks about them fairly casually in English (Paarthurnax, 000BBEAA, 000BBEA8) which may have something to do with the divine nature of dragons (as Lesser Aedra) and the inherent connection between the Elder Scrolls and the creation of Mundus itself. As it stands, we don't have enough information to really know for sure, so it's more or less open to interpretation.

Update: I just remembered about a line in which Alduin refers to an Elder Scroll as "Faal kel."

Faal Kel...?! Nikriinne…
Elder Scroll...? Cowards...
(Alduin, 000CD9DE)

It seems that the usage of the formal form of "the" is more open to interpretation than I previously thought. Perhaps it's primarily used when the speaker believes the subject is worthy of it? That's just speculation, granted, but unfortunately it's about all we've got on it right now.

Hahdremro

It seems that the usage of the formal form of "the" is more open to interpretation than I previously thought. Perhaps it's primarily used when the speaker believes the subject is worthy of it? That's just speculation, granted, but unfortunately it's about all we've got on it right now.

It also seems that's all it can be, barring the possibility that dragon culture has a complete built-in list of things that inspire reverence.

Keep in mind that the official translation is "And the Scrolls have foretold," which means that the intention is to signify more than one scroll. The flexibility of plural nouns in Dovahzul doesn't always provide perfect clarity, unfortunately.

The Scrolls in this instance could mean all of the kel have this prophecy, or it could mean something else entirely. The nature of the Elder Scrolls is beyond mortal comprehension, and such prophecies are often incomplete upon their first viewing. Once the events of 4E 201 (the game Skyrim) have come to pass, the prophecy solidifies and is visible within all of the scrolls as true fact rather than possibility, as happens with all of their prophecies that come true. (Source)

Consider one of the lines of the Song of the Dragonborn: "Hearken now, sons of snow, to an age, long ago, and the tale, boldly told, of the one!" This means that the song could be seen as having been written from a future perspective, despite canonically (and paradoxically) being a ancient song. (Source) Perhaps the prophecies in the Elder Scrolls are, like the kelle themselves, beyond the reach of time, and therefore cannot be properly understood by mortals? Whatever the case, the Elder Scrolls will remain one of the most enigmatic aspects of the series for which they are named.

Late to the conversation but thought I'd share some thoughts. Something to note is that the "Dragonborn" song was one of the first, if not the first thing Bethesda wrote in the dragon language. Given the rhyming, it seems likely that much of its vocabulary was created solely for the song.

Notice that fin is featured 11 times in the song and faal only once. Fin is also used a couple times in association with nouns that the English translation considers to be proper ("the Scrolls," "the Voice") while faal is used in association with a noun that the English translation doesn't consider proper ("the sun").

I think it's likely that fin Kel and faal krein are used in the song simply because they flowed better, and that it wasn't until after the song was written that a distinction between fin and faal was made. So the in-universe explanation is, as Frinmulaar pointed out, that Nords didn't quite know what they were doing.