Learn Hardcore Icelandic: Launin hennar eru góð, en launin hans eru lægri. - Her salary is good, but his salary is lower.

Laun is a plural-only neuter noun (pluralia tantum) meaning pay/salary/wages. Because the subject is grammatically plural, the verb vera must agree: eru (are), not er (is).

What gender is laun, and how does that affect góð?

Laun is neuter plural. Predicative adjectives agree in gender and number, so the strong neuter plural of góður is góð. Hence: Launin … eru góð.

Should it be góðu because launin is definite?

No. After vera (to be), adjectives are predicative and take the strong endings, which do not mark definiteness: góð. The weak form góðu appears when the adjective is attributive before a definite noun: góðu launin hennar (the good wages of hers).

Why do the possessives come after the noun: launin hennar / launin hans?

In Icelandic the normal placement of possessives like hennar (her) and hans (his) is after the noun. This also allows the noun to carry the suffixed definite article: laun-in hennar, laun-in hans.

Can I say hennar laun or hans laun instead?

Yes. Fronting the possessive is possible for emphasis or style, but then the noun typically does not take the suffixed article:

Neutral/ordinary: launin hennar, launin hans
Emphatic/fronted: hennar laun, hans laun Do not mix them as hennar launin.

What are hennar and hans grammatically?

They are independent genitive pronouns meaning her and his. They do not change to match the gender or number of the thing owned. Examples:

hennar bíll, hennar laun, hennar börn
hans bíll, hans laun, hans börn

When would I use the reflexive possessive sinn/sín/sitt instead?

Use the reflexive to refer back to the subject of the same clause; use hans/hennar to refer to someone else.

Hún segir að launin sín séu góð. (She says her own wages are good.)
Hún segir að launin hans séu góð. (She says his wages are good.) In the original sentence there is no reflexive relationship, so hennar/hans is correct.

Why is lægra not used? Why lægri?

Lægri is the comparative of lágur (low) used for all plural nominatives, including neuter plural. Lægra is neuter singular. Compare:

Þetta er lægra. (This is lower — neuter singular.)
Launin hans eru lægri. (His wages are lower — neuter plural.)

What does en mean here? Is it the same word used for than?

Yes. En can mean both but and than. Context decides:

Contrast: Launin hennar eru góð, en launin hans eru lægri. (but)
Comparison: Launin hennar eru hærri en launin hans. (than)

Could I make it one comparative sentence instead of two clauses?

Yes: Launin hennar eru hærri en launin hans. That uses a single clause with a comparative plus en.

Why is there a comma before en?

Because en links two independent clauses. Icelandic typically places a comma before coordinating conjunctions that join full clauses.

Do I have to use the definite form launin, or can I say just laun?

Both occur. Use launin when the wages are specific/known; use bare laun for a more general statement.

Specific: Launin hans eru lægri.
General: Laun hans eru lægri.

How does laun decline?

It is plural-only. Core forms:

Nominative/Accusative plural: laun
Dative plural: launum
Genitive plural: launa
Definite forms: launin (Nom/Acc), laununum (Dat), launanna (Gen)

Is góð the most idiomatic choice for wages, or should I say high/low?

Both are idiomatic. Góð means good/satisfactory. For a more quantitative feel, use high/low:

Launin hennar eru há. (Her wages are high.)
Comparatives: hærri/lægri with en: Launin hennar eru hærri en launin hans.

Any quick pronunciation tips for the tricky vowels?

laun: au is one vowel, roughly like German eu; think something like löy-n.
læ- in lægri: æ sounds like the vowel in English eye.
en: short e, like English ten.
hennar/hans: pronounce the initial h; double n is short.