Learn Hardcore Icelandic: Veitingastaðurinn sem við borðuðum á var rólegur.

Icelandic allows (and, with sem, typically requires) preposition stranding in relative clauses. The preposition stays with the verb phrase instead of moving in front of sem. So you say sem við borðuðum á, not something like á sem. A very natural alternative that avoids stranding is to use the locative relative adverb: Veitingastaðurinn þar sem við borðuðum var rólegur (“the restaurant where we ate was quiet”).

Can I say something like “the restaurant at which we ate,” i.e., á sem við borðuðum?

No. In modern Icelandic, sem is a complementizer (not an inflected relative pronoun), so a preposition cannot directly precede it. You either strand the preposition (sem við borðuðum á) or switch to the “where”-construction (þar sem við borðuðum).

What’s the word order inside the sem-clause? Could it be borðuðum við á?

Inside a sem-clause, the regular embedded word order is Subject–Verb–(Object/PP). So you get sem við borðuðum á. The inversion borðuðum við is used for main-clause questions (and some special embedded V2 contexts), not here.

What case does the missing noun have after á?

Dative. With a static location, á governs the dative. If you reinserted the noun, you’d say á veitingastaðnum (“at the restaurant,” dative singular).

Why is the adjective rólegur in that form? Could it be rólegt or róleg?

Predicate adjectives agree with the subject in gender, number, and case. Veitingastaðurinn is masculine singular nominative, so the adjective is rólegur (masc. nom. sg.). With a feminine subject you’d get róleg, and with a neuter subject rólegt.

What does the ending -inn in veitingastaðurinn do?

It’s the suffixed definite article, roughly “the.” Indefinite: veitingastaður (“a restaurant”). Definite: veitingastaðurinn (“the restaurant”). Icelandic usually marks definiteness by attaching the article to the noun.

What exactly is var?

It’s the past tense of vera (“to be”), third person singular: var = “was.” So the main clause says “(The restaurant) was quiet.”

Could I use í instead of á to mean “at the restaurant”?

The idiomatic choice is á veitingastað (“at a restaurant”). Í (“in”) is possible if you want to emphasize being inside the physical space, but with this noun the default natural choice is á.

Can I drop the subject við in the relative clause?

No. Icelandic is not a pro‑drop language in the standard sense, so you normally must include subject pronouns. sem borðuðum á would be ungrammatical; you need sem við borðuðum á.

What form is borðuðum, exactly?

It’s the 1st person plural past tense of borða (“to eat”): “we ate.” Present would be við borðum (“we eat”). So við borðuðum = “we ate.”

Is borða always transitive? There’s no object here.

It can be used both transitively and intransitively. Here it’s intransitive with a location phrase: “we ate (there/at that place).”

Is a comma needed before sem in this sentence?

No. This is a restrictive relative clause (it identifies which restaurant), and Icelandic does not use a comma before sem in that case. A nonrestrictive clause (extra information) could be set off by commas, but that would change the meaning.

How would I avoid the stranded preposition without using þar sem?

You generally can’t “pied‑pipe” the preposition with sem. The idiomatic non‑stranding alternative for locations is precisely þar sem: Veitingastaðurinn þar sem við borðuðum… (literally “the restaurant where we ate”).

How do I pronounce the special letters here, like ð and á?

ð (in veitingastaðurinn, við, borðuðum) is like the voiced th in English “this.”
á is a diphthong roughly like the “ow” in “cow.”
A rough guide: Vei-ting-a-sta-ður-inn sem vith bor-thu-thum ow var roh-leh-gur. (Icelandic has trilled/flapped r’s; this is only an approximation.)

Could I use átum instead of borðuðum?

átum is the past plural of éta, a stronger/colloquial verb for “eat” (“eat/devour”). It’s fine in speech, but borða is the neutral everyday verb and the safest choice in standard usage: sem við borðuðum á.

Why is sem always sem? Doesn’t it change like “who/whom/which”?

Sem is invariable; it doesn’t inflect for case, gender, or number. The case is determined by the role of the missing element inside the clause (here, dative after á, as noted), but sem itself never changes.