Learn Hardcore Icelandic: Hádegismaturinn hjá háskólanum er oftast ódýr. - The lunch at the university is usually cheap.

hádegismatur + inn → hádegismaturinn
= the lunch
háskóli + inn → háskólinn (nominative)
but in the dative definite it becomes háskólanum (see below)

So -inn / -num / -nir / -nar / -in etc. are different forms of the definite article, depending on case, number, and gender, and they are stuck onto the end of the noun.

What exactly does hádegismaturinn consist of?

hádegismaturinn is a compound word with a definite article:

hádegi = noon, midday
in compounds this often becomes hádegis- (a genitive/“linking” form)
matur = food, meal
hádegismatur = lunch (literally noon‑meal)
hádegismaturinn = the lunch

So the whole thing literally means the lunch, but normally in context it’s understood as the lunch (served) at midday.

Why is there an s in hádegismaturinn, when the basic word is hádegi?

The s is a typical linking element that comes from the genitive form of the first noun in a compound.

Base form: hádegi
Genitive singular: hádegis
Compound: hádegis + matur → hádegismatur

This s-linking is extremely common in Icelandic compounds (similar to things like Arbeitsamt in German, but even more systematic).

What case is háskólanum, and why is that form used here?

háskólanum is dative singular definite.

The preposition hjá always takes the dative case, so the noun that follows must be in dative:

Base noun: háskóli (a university)
Dative singular (indefinite): háskóla
Dative singular (definite): háskólanum

So the pattern is:

Nom. def.: háskólinn (the university, as subject)
Dat. def.: háskólanum (to/at/with the university – after prepositions like hjá)

What does the preposition hjá mean here, and how is it different from í or á?

hjá roughly means at, by, or with, but often in the sense of “at someone’s place / under someone’s responsibility / at an institution”.

In this sentence:

hjá háskólanum = at the university (in the sense of run by / belonging to the university)

Comparisons:

í háskólanum = in the university (physically inside the buildings)
á háskólanum can also be at the university, but more as a location (on campus), and usage can vary by dialect and context.
hjá háskólanum emphasizes associated with / at the institution of the university, not just physically inside it.

So hádegismaturinn hjá háskólanum is like saying the lunch provided by the university / the university lunch.

Why is it oftast and not just oft?

Both are correct Icelandic, but they differ slightly in meaning:

oft = often, frequently
oftast = most often, usually

In practice, oftast is very commonly used to mean usually / generally / in most cases, which is close to English usually.
If you said:

Hádegismaturinn … er oft ódýr.
→ The lunch is often cheap. (many individual occasions)

With oftast:

Hádegismaturinn … er oftast ódýr.
→ The lunch is usually cheap; the normal pattern is that it’s cheap.

Why is ódýr at the end of the sentence and not before hádegismaturinn like in English cheap lunch?

Here ódýr is used as a predicate adjective after the verb er (is), not as an adjective directly modifying the noun.

Predicate structure: [subject] + er + [adjective]
→ Hádegismaturinn … er ódýr. = The lunch is cheap.

If you wanted cheap lunch as a phrase (like a product name), you could put the adjective before the noun:

ódýr hádegismatur = a cheap lunch

So Icelandic word order here is actually parallel to English the lunch is usually cheap; both languages put the adjective after to be / vera in this kind of sentence.

Why is the adjective ódýr in that exact form? Could it be something like ódýrt instead?

Adjectives in Icelandic agree with the noun in gender, number, and case, even in predicate position (after er).

hádegismaturinn is masculine singular nominative.
So ódýr appears as masculine singular nominative: ódýr.

Other forms of ódýr:

Masculine nom. sg.: ódýr
Feminine nom. sg.: ódýr (same spelling, different declension pattern in other cases)
Neuter nom. sg.: ódýrt

You’d use ódýrt if the subject were a neuter noun in the nominative, e.g.:

Verðið er oftast ódýrt.
(The price is usually cheap. – verð is neuter.)

In this sentence the subject is masculine (hádegismaturinn), so ódýr is the correct matching form.

Is the word order fixed, or can I move hjá háskólanum or oftast around?

Icelandic word order is flexible, especially for adverbials and prepositional phrases, though there are preferences.

All of these are possible and natural, with slightly different emphasis:

Hádegismaturinn hjá háskólanum er oftast ódýr.
(neutral; standard order)
Hádegismaturinn er oftast ódýr hjá háskólanum.
(slight emphasis on at the university as the location where it’s cheap)
Oftast er hádegismaturinn hjá háskólanum ódýr.
(emphasizes usually – good in written style or careful speech)

What you normally keep is:

The finite verb (er) in second position in main clauses.
The subject close to the verb unless you’re intentionally fronting something for emphasis.

Everything else (time, place, manner) can be moved around more freely.

Could I use venjulega instead of oftast? What’s the difference?

Yes, you can, but there is a nuance difference:

oftast = most often, usually (literally a superlative of oft)
venjulega = usually, as a rule, normally (from venjulegur = usual/normal)

In many everyday contexts, they can both translate as usually:

Hádegismaturinn hjá háskólanum er venjulega ódýr.
→ The lunch at the university is usually cheap.

oftast subtly highlights frequency (most of the time it happens),
venjulega highlights normality (that’s the normal state of things).
But for most learners, they are very close in day‑to‑day use.

Is hádegismatur the only way to say lunch, or could I say something like matur í hádeginu?

Both are possible:

hádegismatur is the standard single word for lunch.
matur í hádeginu literally = food at noon, also understandable as lunch, but it feels more descriptive/explicit.

In this sentence, the natural idiomatic choice is hádegismaturinn.
You might hear matur í hádeginu in contexts like:

Við fáum góðan mat í hádeginu.
→ We get good food at lunchtime.

So as a learner, use hádegismatur as your default word for lunch.

How do I pronounce tricky parts like há, ð, and é in this sentence?

Some quick pronunciation tips (approximate for English speakers):

há‑ in hádegis‑ and háskólanum:
á is like a long ow in now, but a bit tenser: [hau].
ð in hádegis‑:
Pronounced like th in this, not like d. So hádegi starts like HOW-they‑….
é in háskólanum?
Actually, there is no é in this sentence; but when you do see é in other words, it is usually like ye in yes (e.g. ég ≈ yeg).
ó in háskólanum:
This is a long o, similar to British go, but tenser: [oː].
Stress:
Icelandic almost always stresses the first syllable of each word:
HÁ‑degis‑ma‑tur‑inn, HJÁ HÁ‑skó‑la‑num, ER ÓF‑tast Ó‑dýr.

Listening to native audio is very helpful, because Icelandic spelling‑to‑sound rules are regular but quite different from English.