@Sninguistics, that's an interesting question. On the surface of it, existed or been there seem much better translations. Duo lists finns as a deponent verb (https://www.duolingo.com/comment/6094592), meaning there is no version without the -s.
And yet, you're right in a sense: there is a verb att finna, according to https://en.wiktionary.org/wiki/finna#Swedish, meaning to find, and whose passive past perfect form is funnits. But, if I understand correctly about deponent verbs, that is not the same word (semantically), it just happens to be spelt (and presumably pronounced) the same.
So does that make this sentence ambiguous? It would be interesting to hear an answer from a native speaker.
(In many cases, it seems there is not a lot of semantic distance between been found and existed, but consider for example "Noah's ark has never been found" versus "Noah's ark has never existed" There are some people who would readily agree with the first statement but would utterly refute the second!)
Much obliged for your response - this is precisely why I asked. att ha hittats is probably better for the meaning 'to have been found', but I figure that att ha funnits could also mean the same thing. The way I understand it is that in most languages with deponent verbs, if for a deponent verb there exists a very similar active verb (like att finna and att finnas) then the two are probably related somehow. Plus it makes a lot of sense to me to think that att finnas came from the meaning of 'to be found' (considering that existence is usually connotative of needing to be found, somehow or the other).
In context of this sentence, though, the main reason I asked is because 'it had never been found' feels more usable to me than 'it had never existed'.
But I would like for a Native speaker to weigh in. Is this acceptable?
finnas and funnits are very tied to the deponent meaning, so I think it would be hard to make it passive even if one wanted to. I would never interpret this sentence that way, and I would say blivit funnen if I wanted to say that. (there's also the verb återfunnits which could work in some cases).
Native speaker, University of California Linguistics grad (UCSB 1978)
'It had never been found' would refer to a search sans success. 'Att finnas' has less baggage to bear, and usually implies no more than 'is,' 'funnits' being 'been.'
My choice of 'It had never existed' or 'It had never been' would be made on semi-aesthetic factors led by the rest of the discourse, the second is slightly more elevated but would be bypassed for the former unless required by register or good taste, such as to buffer an otherwise overly Latinized passage.
My understanding (based solely on my DL experience) is that den is used when it's referring to a specific -en noun. For example, perhaps the question being answered here is something like "Hittade du boken jag letade efter?". (Someone please correct me if I'm misstating anything or if my Swedish is wrong!)
You're right. If it refers to a specific noun, as it does here, both den and det work depending on the gender of that noun.
If on the other hand det is a placeholder prounoun, it does not change. For instance in the typical det är construction: det är en bil or det är ett hus, it's always det because that det does not refer to the car or house.
Your sentence should be Hittade du boken jag letade efter? 'Did you find the book I was looking for?' – if you really want to use the pluperfect, that would be Hittade du boken jag hade letat efter? but that's a little less likely – 'Did you find the book I had looked for?'
Present is "finns", infinitive is "finnas", past is "fanns". I recommend using Wiktionary for this sort of thing (it lists the conjugations and often gives etymologies, which can be helpful).
There's not much that isn't in either Wiktionary or Folkets Lexikon, at least for what I've encountered so far.