Hún vill ekki hafna boðinu of fljótt, því henni finnst fólkið þar mjög vinalegt.

Because finnast uses a different construction from ordinary to be or to think in English.

In henni finnst fólkið þar mjög vinalegt, henni is in the dative case and means to her. A very literal sense is:

To her, the people there seem very friendly.

So:

• hún = she (nominative)
• henni = to her / her (dative)

This dative experiencer pattern is very common with finnast:

• Mér finnst þetta gott. = I think this is good.
• Honum finnst það skrýtið. = He finds that strange.

Finnst is the present singular form of finnast.

In this kind of sentence, finnast means something like:

• to seem
• to feel
• to be found / considered by someone

So henni finnst fólkið þar mjög vinalegt means literally something close to:

It seems to her that the people there are very friendly.

It is not the same as the ordinary verb finna in the straightforward sense to find something physically.

A very common Icelandic pattern is:

• Mér finnst... = I think... / I find...
• Þér finnst... = You think...
• Henni finnst... = She thinks...

Because hafna takes the dative case.

The noun is boð = invitation / offer. Its definite form is:

• nominative/accusative: boðið = the invitation
• dative: boðinu = to the invitation / the invitation after a dative-governing verb

Since the sentence has hafna boðinu, the verb hafna requires the object to be dative.

So:

• hafna boðinu = decline/reject the invitation

This is something Icelandic learners have to memorize with certain verbs: the verb tells you what case the object must take.

Yes, hafna is a verb that normally governs the dative.

Examples:

• hafna tillögunni = reject the proposal
• hafna beiðninni = deny the request
• hafna boðinu = decline the invitation

So when you learn hafna, it is best to learn it as:

• hafna + dative

Because vill is the finite verb in the main clause, and ekki usually comes after the finite verb in ordinary Icelandic word order.

So:

• Hún vill ekki hafna...

breaks down as:

• Hún = subject
• vill = finite verb
• ekki = not
• hafna = infinitive

This is very normal with modal verbs:

• Ég vil ekki fara. = I do not want to go.
• Hann getur ekki komið. = He cannot come.

Because Icelandic modal verbs usually take a bare infinitive, without að.

So:

• vil fara = want to go
• get komið = can come
• skal gera = shall do
• vill hafna = wants to decline

English uses to before the infinitive, but Icelandic often does not use að after modal verbs.

No. Icelandic of here means too, not English of.

So:

• fljótt = quickly
• of fljótt = too quickly / too soon

That gives:

• ekki hafna boðinu of fljótt = not decline the invitation too quickly

Compare:

• mjög fljótt = very quickly
• of fljótt = too quickly

So of expresses excess, just like too in English.

Because fólk in Icelandic is a collective noun. Grammatically, it is singular neuter, even though its meaning refers to multiple people.

So:

• fólk = people, folk
• fólkið = the people

But grammatically it behaves like a singular neuter noun. That is why other words around it also appear in singular neuter form.

This is similar to how English sometimes treats collective nouns specially, though Icelandic does it more systematically here.

Because vinalegt agrees with fólkið, and fólkið is neuter singular.

The adjective must match the noun in gender, number, and case. Since fólkið is grammatically neuter singular, the adjective is also neuter singular:

• vinalegur = masculine singular
• vinaleg = feminine singular
• vinalegt = neuter singular

So:

• fólkið ... vinalegt = the people ... friendly

Even though the meaning is plural in English, the grammar is singular neuter in Icelandic.

Yes, in this sentence því means because / for.

So the second part:

• því henni finnst fólkið þar mjög vinalegt

means:

• because she finds the people there very friendly

This því is a conjunction connecting the reason to the first clause.

A learner should know that því can have other uses in Icelandic too, but here its job is simply to introduce the reason.

Yes, very often you can.

A more everyday version could be:

• Hún vill ekki hafna boðinu of fljótt, af því að henni finnst fólkið þar mjög vinalegt.

Both mean because, but af því að is often felt to be more conversational and explicit, while því can sound a bit more compact or written.

Þar means there.

In this sentence, it goes with fólkið:

• fólkið þar = the people there

So the meaning is not just the people, but the people in that place / at that event / there.

Its placement is natural Icelandic word order. Icelandic often puts adverbs like þar after the noun phrase they relate to.

A useful breakdown is:

• Hún = subject, she
• vill = finite verb, wants
• ekki = negation, not
• hafna = infinitive, to decline / reject
• boðinu = dative object, the invitation
• of fljótt = adverb phrase, too quickly
• því = because
• henni = dative experiencer, to her
• finnst = seems / feels / finds
• fólkið = grammatical subject, the people
• þar = there
• mjög = very
• vinalegt = adjective agreeing with fólkið, friendly

A very literal reading is:

She wants not to decline the invitation too quickly, because to her the people there seem very friendly.

That literal structure helps explain the cases and word order.