Learn Hardcore Icelandic: Hún er hljóðlát í vinnunni, en talar mjög hátt þegar hún syngur með vinum sínum. - She is quiet at work, but speaks very loudly when she sings with her friends.

Hún is 3rd person singular feminine, so the adjective must be in the feminine singular form: hljóðlát.
Hljóðlátur would be masculine singular.
Hljóðlátir would be masculine plural.

In Icelandic, adjectives agree with the noun (or pronoun) in gender, number, and case, even when they appear after er (“is”).

What is the difference between í vinnunni and just í vinnu?

Both can translate as “at work”, but there is a nuance:

í vinnunni = literally “in the work / in the workplace,” with the definite form of vinna (vinnan → vinnunni in dative). This often refers to the place (at her job / at her workplace).
í vinnu = literally “in work,” with no article, often focusing on the state of being employed or working, not a specific workplace.

So hljóðlát í vinnunni implies she is quiet at her workplace.

Why is there a comma before en? Is that required?

Yes, Icelandic normally uses a comma before en when it introduces a contrastive clause, similar to English “but”.

Hún er hljóðlát í vinnunni, en talar mjög hátt…
The comma separates two independent clauses with a contrasting relationship: quiet at work but loud when singing with friends.

Why is it talar mjög hátt and not something like talar mjög hár?

Hátt is the adverbial form related to the adjective hár (“high, loud”).

hár = adjective (“high / tall / loud” as a describing quality of a noun)
hátt = used as an adverb meaning “loudly / in a loud way”

In Icelandic, many adjectives form adverbs by using the neuter singular accusative form, which here is hátt. So after talar (“speaks”), you need the adverb: talar mjög hátt = “speaks very loudly.”

What is the role of mjög in mjög hátt? Where does it usually go?

Mjög means “very” and it functions as an intensifier.

It normally comes before the adjective or adverb it modifies: mjög hátt, mjög fallegur, mjög vel, etc.
So talar mjög hátt literally means “speaks very loud(ly).”

Why is it þegar hún syngur and not þegar syngur hún?

In Icelandic:

Main clauses normally have the verb in second position (V2): e.g. Í vinnunni er hún hljóðlát.
Subordinate clauses introduced by words like þegar (“when”), að, af því að normally do not use V2; the verb goes later in the clause, like in English.

So in a subordinate clause:

Correct: þegar hún syngur (“when she sings”)
Marked/weird here: þegar syngur hún (this would sound like you’re emphasizing or turning it into a sort of question-like structure).

Why is it með vinum sínum and not með vini sínum?

Because she is singing with friends (plural), not just one friend.

vinur (friend) → dative plural: vinum
með takes the dative case here, so “with (her) friends” becomes með vinum sínum.

If it were just one friend:

dative singular: vini → með vini sínum = “with her (one) friend.”

Why is vinum in the dative case in með vinum sínum?

The preposition með (“with”) usually governs the dative case when it means “together with” someone or something.

So:

nominative: vinir (friends)
dative plural: vinum

Since it’s “with (her) friends,” and með + person/thing = dative, we get með vinum sínum.

Why is it sínum and not hennar in með vinum sínum?

Sínum is the reflexive possessive pronoun, referring back to the subject of the clause, here hún.

vinum sínum = “her own friends” (the subject’s friends).
vinum hennar = “her friends,” but could mean some other woman’s friends, not necessarily the subject’s.

Icelandic strongly prefers sinn/sín/sitt/sinir/sínar/sín (here sínum, dative plural masculine) when the possessor is the subject of the same clause.

What form is sínum exactly, and why that ending?

Base form: sinn (reflexive possessive). In með vinum sínum it must agree with vinum in case, number, and gender:

vinum: dative, plural, masculine
So sinn becomes sínum: dative plural masculine

The matching is: vinum (dat. pl. m.) sínum (dat. pl. m.).

How is the verb syngur formed from að syngja?

Að syngja (“to sing”) is a strong verb with stem changes. The 3rd person singular present is:

ég syng – I sing
þú syngur – you sing (sg.)
hann / hún / það syngur – he/she/it sings

So hún syngur is simply the normal 3rd person singular present form of að syngja.

Can hún be dropped, like in some other languages (“Is quiet at work…”)?

No. Icelandic is not a “null-subject” (pro-drop) language in the same way as Spanish or Italian.

You normally need to state the subject pronoun:

Hún er hljóðlát í vinnunni… is correct.
Leaving out hún would sound ungrammatical or extremely unnatural in standard Icelandic.

Is hljóðlát the same as róleg or þögul, or are there differences?

They are related but not identical:

hljóðlát = quiet, not making much noise; often about speaking softly or not talking much.
róleg = calm, relaxed, not agitated; more about mood/behavior than volume.
þögul = silent, not speaking at all or almost not at all.

In this sentence, hljóðlát í vinnunni suggests she speaks softly or doesn’t talk much at work, not that she is completely silent.