Learn Hardcore Icelandic: Við tölum hljóðlátara í bókasafninu.

In Icelandic, comparative adverbs are often identical in form to the masculine accusative singular of the comparative adjective. So hljóðlátara here functions adverbially, modifying tölum (we speak).

Why do we say hljóðlátara and not use something with meira, like meira hljóðlátur?

Icelandic usually forms the comparative of regular adjectives by adding endings to the adjective itself, not by using meira (more) as in English or some other languages.

For hljóðlátur:

Positive: hljóðlátur – quiet
Comparative: hljóðlátari / hljóðlátara – quieter, more quiet
Superlative: hljóðlátastur – quietest

So the natural way to say more quietly with this word is with hljóðlátara, not meira hljóðlátur.
Meira + adjective is used, but mainly with certain adjectives (especially longer or irregular ones); hljóðlátur has its own regular comparative form.

Why is there no -t at the end (like hljóðlátt) if it is used like an adverb?

In the positive degree, Icelandic adverbs are often identical to the neuter singular form of the adjective:

hljóðlátur – quiet (masc.)
hljóðlátt – quietly (adverb)

So hljóðlátt is the basic adverb: quietly.

In the comparative, the adverb typically takes the masculine accusative singular form:

hljóðlátari – (masc. nom. sg. comparative)
hljóðlátara – (masc. acc. sg. comparative) → used adverbially

So:

hljóðlátt – quietly
hljóðlátara – more quietly

That is why you see -ara instead of -t in the comparative.

What case is bókasafninu in, and why?

Bókasafninu is in the dative singular with the definite article attached.

Base (nominative singular, indefinite): bókasafn – a library
Dative singular, definite: bókasafninu – in the library

The preposition í (in) can take accusative or dative:

í
- accusative = movement into something
  - e.g. Ég fer í bókasafnið. – I go into the library.
í
- dative = location (being in something)
  - Við tölum hljóðlátara í bókasafninu. – We speak more quietly in the library.

Here it is a location, so í governs the dative, giving bókasafninu.

What is the base form and structure of bókasafninu?

Strip the endings and article:

bókasafninu → bókasafni (dative, no article) → bókasafn (base form)

Bókasafn is a compound:

bók – book
safn – collection, museum, gathering

So bókasafn literally means book-collection, i.e. library.

Declension (neuter, singular, indefinite):

Nom.: bókasafn – (a) library
Acc.: bókasafn
Dat.: bókasafni
Gen.: bókasafns

Add the definite article as an ending:

Nom.: bókasafnið – the library
Acc.: bókasafnið
Dat.: bókasafninu
Gen.: bókasafnsins

Why is it í bókasafninu and not á bókasafninu?

Both í and á can sometimes translate as in / at, but Icelandic makes a clearer distinction:

í is mainly inside something: in a room, in a building, in a country.
á is on or at a surface or certain types of places (workplaces, events, etc.): on the table, at the office, at the concert.

For being inside a building like a library, í is the normal choice:

í bókasafninu – in the library (literally inside)
á bókasafninu would sound more like on the library (on top of it), which is not what you want here.

Can the word order change? For example, can I say Í bókasafninu tölum við hljóðlátara?

Yes, that word order is correct and quite natural.

Icelandic main clauses are generally verb-second (V2):

If the sentence starts with the subject:
- Við (subject) – tölum (verb) – hljóðlátara í bókasafninu
If you move a different element to the front (here, the place expression):
- Í bókasafninu (fronted element) – tölum (verb) – við (subject) – hljóðlátara

Both are fine:

Við tölum hljóðlátara í bókasafninu.
Í bókasafninu tölum við hljóðlátara.

What must stay stable is that in a normal statement the finite verb (tölum) should be second in the clause.

Can I drop við and just say Tölum hljóðlátara í bókasafninu?

In normal statements, Icelandic does not normally drop the subject pronoun the way Spanish or Italian does. You usually keep við:

Við tölum hljóðlátara í bókasafninu. – standard

However:

In imperatives (commands), you do usually drop the subject:
- Tölum hljóðlátara í bókasafninu. can be understood as Let’s speak more quietly in the library.
- Here it is an inclusive let’s form, not a simple statement.

So:

As a statement about what we do: keep við.
As a suggestion/command (let’s…): you can drop við and start with Tölum.

How do I pronounce the tricky parts: við, tölum, hljóðlátara, bókasafninu?

Approximate pronunciations (using English-like hints):

við ≈ vith
- ð is like th in this.
- IPA: /vɪð/
tölum ≈ tuh-lum
- ö is like the vowel in British bird or French eu (as in peur).
- t is aspirated (a little puff of air).
- IPA: /ˈtʰœːlʏm/
hljóðlátara ≈ hlyo-th-low-ta-ra
- hl is a voiceless l sound; you slightly blow air before l; often sounds like a breathy ly.
- jóð: j like English y, ó like English o in go, ð like th in this.
- Main stress on the first syllable: HLJÓÐ-lá-ta-ra.
- IPA (approx.): /ˈl̥jouðˌlauːtara/
bókasafninu ≈ bo-ka-saf-ni-nu
- ó as in go.
- fn often has a slight p-like element for many speakers, something like fpn.
- Stress on the first syllable: BÓ-ka-saf-ni-nu.
- IPA (approx.): /ˈpouːkaˌsapnɪnʏ/

These are still approximations; listening to native audio is very helpful for fine details.

How would I say We don’t speak more quietly in the library?

You negate the verb with ekki, keeping the verb in second position:

Við tölum ekki hljóðlátara í bókasafninu.

Structure:

Við – subject (we)
tölum – finite verb (speak)
ekki – not
hljóðlátara – more quietly
í bókasafninu – in the library

How would I say Please speak more quietly in the library (talking to a group)?

Use the plural imperative of tala:

Second person plural imperative: talið! (speak!)
Add the adverb and place phrase:

Talið hljóðlátara í bókasafninu. – Speak more quietly in the library.

To make it more polite, you can add vinsamlegast (please):

Vinsamlegast talið hljóðlátara í bókasafninu.