Learn Hardcore Icelandic: Ostur er aðeins á tilboði í dag.

Icelandic has no separate indefinite article. Bare ostur is the indefinite nominative singular (“cheese” in general or an unspecified cheese). Osturinn means “the cheese,” used for a specific, known cheese. Both forms are correct depending on context.

What does á tilboði literally mean, and why is it tilboði with -i?

á tilboði is the set phrase “on sale / on special.” The preposition á takes the dative when describing a state/location. tilboð (“offer, special”) is neuter; its dative singular is tilboði. So you say á tilboði, not “á tilboð.”

Is á tilboði the same as í boði?

No. á tilboði = “on sale (discounted).” í boði = “available; on offer (as an option),” with no discount implied. Example: Kaffi er í boði (“Coffee is available”) vs. Kaffi er á tilboði (“Coffee is on sale”).

How is á tilboði different from á útsölu or á afslætti?

á tilboði: product-specific special price, typically short-term.
á útsölu: on (clearance) sale, often storewide/seasonal (common in clothing/retail).
á afslætti / með afslætti: at a discount (neutral wording about reduced price).

Why is it í dag and not dagur or degi?

With time expressions, í takes the accusative: dagur (nom) → dag (acc). í dag is the fixed phrase “today.” The dative degi is used for static location (“in the day”), not for time like this.

Can I move aðeins around? Where does “only” belong?

Yes—word order is flexible for emphasis, but the finite verb must stay second in main clauses.

Neutral: Ostur er aðeins á tilboði í dag.
To make “only today” explicit: Ostur er á tilboði aðeins í dag. or Aðeins í dag er ostur á tilboði.
To front the time: Í dag er ostur aðeins á tilboði. As a rule, aðeins tends to modify what immediately follows.

Does aðeins always mean “only”?

Usually, yes. It also works as a softener meaning “just / a little” in requests: Geturðu aðeins hjálpað mér? = “Could you help me a bit?” Synonyms: bara (very colloquial), einungis and eingöngu (more formal).

What case does á take—dative or accusative?

Both, depending on meaning:

State/location → dative: Ostur er á tilboði; Bókin er á borðinu.
Motion onto → accusative: Settu ost á borðið (“Put cheese on the table”).

Could I say Ostur er aðeins í boði í dag?

You can, but it changes the meaning to “Cheese is only available today,” with no implication of a discount. For a reduced price, use á tilboði.

How do I say it if more than one kind of cheese is on sale?

Use the plural: Ostar eru á tilboði í dag. To say “all cheeses,” you can use Allir ostar eru á tilboði í dag, or the mass-singular style common on signs: Allur ostur er á tilboði í dag.

Why er and not eru?

er is the 3rd‑person singular of vera (“to be”), agreeing with singular ostur. With plural ostar, you use eru: Ostar eru á tilboði í dag.

Can the verb er be dropped, like on store signs?

Yes. Headlines/signage often omit it: Ostur á tilboði í dag. In full sentences, keep er.

Pronunciation tips for tricky bits?

ð (in aðeins, tilboði) = voiced “th” as in “this.”
á = “ow” (as in “cow”), fairly short.
í = long “ee.”
Rough guide: Ostur “OS-tur,” aðeins “AH-thense,” á “ow,” tilboði “TIL-bo-thi,” í dag “ee dahg” (the g is soft, like a voiced h).