Learn Hardcore Swedish: Det finns tio tusen böcker i biblioteket. - There are ten thousand books in the library.

finnas = “to exist / to be found.”
Det finns tio tusen böcker i biblioteket = “There exist ten thousand books in the library” → “There are ten thousand books in the library.”

You normally do not say Det är tio tusen böcker i biblioteket for existence.
Det är focuses more on identifying or describing something (“It is…”, “They are…”) rather than stating that something exists or is present somewhere.

Why is det used at the beginning? What does it refer to?

Here det is a dummy (expletive) subject, just like “there” in English in “There are ten thousand books…”

It does not refer to a specific thing.
Swedish, like English, generally requires a subject, so det fills that slot.

So Det finns … is a fixed, very common pattern to introduce the existence of something.

Why is it tio tusen and not one word like tiotusen? Are there rules for writing numbers in Swedish?

Both tio tusen and tiotusen can be seen, but the modern recommended spelling is usually two words for bigger round numbers:

tio tusen = ten thousand
tjugo tusen = twenty thousand
hundra tusen = one hundred thousand

For smaller numbers, you normally write one word:

tio (10)
elva (11)
tolv (12)
tjugotre (23)
fyrtiofem (45)

You may still see tiotusen in some texts, but tio tusen is safe and common.

Why is the plural böcker and not something like boks or bokar?

Böcker is an irregular plural of en bok (a book). The pattern is:

singular indefinite: en bok (a book)
singular definite: boken (the book)
plural indefinite: böcker (books)
plural definite: böckerna (the books)

Notice two things:

The vowel changes: o → ö (bok → böcker).
The ending -er is added in the plural.

Swedish has many irregular plurals like this (with vowel change and -er ending), so you generally have to learn them individually.

Why is there no article before tio tusen böcker, like “the ten thousand books” or “a ten thousand books”?

In Swedish, when you talk about an unspecified number of countable things, you normally use the indefinite plural without an article, just like in English:

tio böcker = ten books
många böcker = many books
några böcker = some books

So tio tusen böcker simply means “ten thousand books”, not “the ten thousand books”.

If you want “the ten thousand books”, you would say:

de tio tusen böckerna (with de and the definite plural ending -na)

Why is it böcker and not böckerna in this sentence?

Böcker is indefinite plural (just “books”), while böckerna is definite plural (“the books”).

Det finns tio tusen böcker i biblioteket.
→ There are ten thousand books (some number of books, not specified as “the known group”).

If you said:

Det finns de tio tusen böckerna i biblioteket.

you’d be talking about those specific, already identified ten thousand books. That would be unusual in a neutral, informative sentence. The original sentence simply states how many books there are, in general.

Why is it i biblioteket and not på biblioteket? Both seem to mean “at the library”.

i and på are both used with places, but they have slightly different nuances:

i biblioteket = in the library, inside the building (physical interior).
på biblioteket = at the library, meaning “at that place / at that institution,” not necessarily focusing on being inside.

In this sentence we are talking about books that physically exist inside the library, so i biblioteket is the natural preposition.

You might say:

Jag pluggar på biblioteket.
I study at the library.

Here the focus is on location as a place where you do something, so på is common.

Why is biblioteket in the definite form? Why not just bibliotek?

Biblioteket = “the library” (definite form).
ett bibliotek = a library (indefinite).
bibliotek (without article) can be plural or used in some special patterns.

In Det finns tio tusen böcker i biblioteket, we are talking about a specific, known library – probably the one both speaker and listener have in mind (for example, the local public library).

Swedish almost always marks definiteness on the noun itself with an ending:

en bok → boken
ett bibliotek → biblioteket

So i biblioteket = “in the library (that we know which one it is).”

Can I say Det är tio tusen böcker i biblioteket instead?

This is not idiomatic Swedish when you want to express existence. Native speakers use:

Det finns tio tusen böcker i biblioteket.

Det är + [plural noun] is used differently, for example:

Det är tio böcker på bordet.
This can be heard, but it often feels more like “Those are ten books on the table” (identifying what you see), or it can be used in counting situations.

For a neutral statement about what exists in a place, Det finns is the default and safest choice. With a large, more abstract quantity like tio tusen böcker i biblioteket, Det finns is strongly preferred.

Can I move the parts around, like I biblioteket finns det tio tusen böcker?

Yes. Swedish word order is flexible for emphasis. These sentences are all grammatical:

Det finns tio tusen böcker i biblioteket. (neutral, very common)
I biblioteket finns det tio tusen böcker. (emphasis on “in the library”)
Tio tusen böcker finns det i biblioteket. (emphasis on the number of books)

All still mean essentially: “There are ten thousand books in the library.”
What changes is which part is highlighted or contrasted in the context.

Can I drop det and just say Finns tio tusen böcker i biblioteket?

You might hear Finns tio tusen böcker i biblioteket in very informal spoken Swedish, especially as a question:

Finns tio tusen böcker i biblioteket? = “Are there ten thousand books in the library?”

But in standard, neutral Swedish, you should include det:

Det finns tio tusen böcker i biblioteket.

Leaving out det is colloquial and not recommended in writing or in careful speech.

What are the forms of the verb finnas, and why is it finns here?

Finnas is the infinitive (dictionary form). It’s an impersonal verb and is usually used with det. Its main forms:

infinitive: att finnas (to exist, to be found)
present: finns (there is / there are)
past (preterite): fanns (there was / there were)
supine: funnits (has existed / have been)

So in the present tense, regardless of number, you use finns:

Det finns en bok. – There is one book.
Det finns tio tusen böcker. – There are ten thousand books.

How do you pronounce böcker and biblioteket, especially the ö and the stress?

Approximate pronunciation:

böcker: [ˈbœkːɛr]
- ö is similar to the vowel in British “bird”, but with more rounded lips.
- Stress on the first syllable: BÖC-ker.
biblioteket: [bɪblɪʊtˈeːkɛt] (simplified)
- Stress is on -te-: bib-li-o-TE-ket.
- The final -et is unstressed and quite short.

English speakers often need to practice:

The ö in böcker (rounded mid-front vowel).
Putting the main stress on the correct syllable: BÖC-ker, bib-li-o-TE-ket.