Learn Hardcore Icelandic: Við fórum einu sinni í bíó.

It is only capitalized because it is the first word of the sentence. In the middle of a sentence it would be við (lowercase), just like English we vs We.

Related forms (for reference):

Nominative: við – we (subject)
Accusative: okkur – us (object)
Dative: okkur
Genitive: okkar – our / of us

You normally cannot drop við in Icelandic (see a later question); Icelandic is not a “pro‑drop” language like Spanish or Italian.

What exactly is fórum, and what is the basic verb it comes from?

Fórum is the past tense, 1st person plural form of the verb að fara (to go).

Very simplified paradigm:

Infinitive: að fara – to go
Present:
- ég fer – I go
- við förum – we go
Past (simple):
- ég fór – I went
- við fórum – we went

So Við fórum … literally = We went …

It’s a strong verb, so the stem vowel changes: a → ó (fara → fór‑).

What is the difference between förum and fórum?

Both are 1st person plural forms of fara, but in different tenses:

við förum = we go / we are going (present)
við fórum = we went (simple past)

So:

Við förum í bíó. – We go to the cinema. / We are going to the cinema.
Við fórum í bíó. – We went to the cinema.

In your sentence, fórum shows that this happened in the past.

What does einu sinni literally mean, and how is it used?

Einu sinni is an adverbial phrase meaning once / one time.

Literally it is “at one time / on one occasion”:

einu – one in the neuter dative form
sinni – dative singular of a noun sinn (a “time, occasion”)

Over time, einu sinni has become a fixed phrase meaning once, and is used a lot:

Við fórum einu sinni í bíó. – We went to the cinema once.
Hann hefur bara gert þetta einu sinni. – He has only done this once.
Einu sinni var… – Once upon a time…

You may also see Einu sinni var… starting fairy tales in Icelandic.

Why is it einu and not einn, eina or eitt?

Because of gender and case.

The numeral einn (one) declines for gender and case. Very briefly:

Masculine: einn (nom), einn (acc), einum (dat)
Feminine: ein (nom), eina (acc), einni (dat)
Neuter: eitt (nom/acc), einu (dat)

In einu sinni:

sinn is a neuter noun
in the phrase einu sinni, it appears in the dative (sinni)
the numeral must agree with it: neuter dative einu

So you get einu sinni, not einn sinni or eina sinni.

Can einu sinni go in a different place in the sentence?

Yes. Icelandic word order is quite flexible for adverbials like einu sinni, as long as the finite verb stays in 2nd position in a main clause.

All of these are grammatical, with slightly different rhythm/emphasis:

Við fórum einu sinni í bíó. (very natural: Subject–Verb–Time–Place)
Við fórum í bíó einu sinni. (also fine: Subject–Verb–Place–Time)
Einu sinni fórum við í bíó. (emphasis on “once”; story-telling style)

What you normally cannot do is move einu sinni in front of fórum while keeping við first, e.g.
✗ Við einu sinni fórum í bíó. – ungrammatical, because it breaks the verb‑second rule.

What does bíó mean, and is it like “cinema” or “the movies” in English?

Bíó is the everyday Icelandic word for cinema / movie theater. In practice it corresponds to English:

the cinema
the movies
the theater (when you mean “movie theater”, not stage plays)

So í bíó is usually translated to the cinema or to the movies, depending on your dialect of English.

There is a more formal word kvikmyndahús, but in everyday speech people overwhelmingly say bíó.

Why is it í bíó and not something with “the”, like í bíóið?

English often says to the cinema / to the movies, but Icelandic frequently leaves the noun indefinite in these generic activity expressions.

Compare:

Við fórum í bíó. – We went to the cinema / to the movies.
(generic: going to see a film)
Við fórum í sund. – We went swimming. (literally “into swimming”)
Við fórum í skóla. – We went to school. (as an activity/institution)

If you say í bíóið, it sounds more like to the specific cinema (building), or “into the cinema”, referring to a particular place that is already known in the context:

Við stóðum úti, svo fórum við inn í bíóið.
We were standing outside, then we went into the cinema (building).

Why do we use í here, and not á?

Both í and á can mean things like in / on / at / to, but they are used with different kinds of nouns and in many fixed expressions.

Very roughly:

í is more “in / into” (inside something, or entering something)
á is more “on / onto / at” (surface, event, or sometimes location)

With bíó the standard idiomatic expression is:

í bíó – to the cinema / in the cinema

Other examples:

í skóla – to school / in school
í vinnu – to work / at work
á veitingastað – at a restaurant
á tónleika – at a concert

Learners mostly need to memorize which preposition goes with which noun in these common expressions. For bíó, it is í.

What case is bíó in here, and how would it change if I meant “in the cinema” instead of “to the cinema”?

The preposition í can take either:

Accusative – for motion into / to a place
Dative – for location in a place

Your sentence expresses motion to the cinema, so í governs the accusative:

Indefinite:
- Nom/Acc sg: bíó
- Dat sg: bíói
With definite article:
- Nom/Acc sg: bíóið – the cinema (subject/object)
- Dat sg: bíóinu – in/to the cinema (as a location)

So:

Við fórum í bíó. – We went to (a/the) cinema. (motion → accusative)
Við vorum í bíóinu. – We were in the cinema. (location → dative)

If you wanted We went into the cinema (building), with a definite reference, you could say:

Við fórum í bíóið. – We went into the cinema. (motion to a specific cinema)

Can I drop við and just say Fórum einu sinni í bíó?

Normally, no. Icelandic is not a pronoun‑dropping language like Spanish or Italian, so you usually must include the subject pronoun:

Við fórum einu sinni í bíó. – normal
✗ Fórum einu sinni í bíó. – incomplete in standard Icelandic

You might see or hear something like Fórum einu sinni í bíó:

in very informal speech when the subject is obvious from context,
in diaries / notes / headlines,
as an answer where við is understood from the question.

But as a full, neutral sentence, you should keep við.

How do you pronounce Við fórum einu sinni í bíó?

Approximate Icelandic pronunciation (IPA):

Við – [vɪð]
v as in vet, i like bit, ð like th in this.
fórum – [ˈfouːrʏm]
ó is a long ow sound ([ou]), u is like a short German ü ([ʏ]).
einu – [ˈeiːnʏ]
ei like ay in day, final u again [ʏ].
sinni – [ˈsɪnːɪ]
Short i as in sit, double nn is a long n sound.
í – [iː]
A long ee sound.
bíó – [ˈpiːjou]
b is closer to an unaspirated p, í is ee, and ó is [ou], here realized as [jou].

Very rough English-like approximation (not perfect, just to get close):

Við fórum einu sinni í bíó ≈ "Vith FOH-rum AY-nu SIN-ni ee BEE-yo"
(with soft th in Við, and stress on fórum, einu, and bíó).

How would I say “We have been to the cinema once” in Icelandic, and is it different in meaning from Við fórum einu sinni í bíó?

A natural present perfect version is:

Við höfum einu sinni farið í bíó.

Breakdown:

höfum – have (1st person plural, from hafa)
faríð – supine of fara (used in perfect tenses)
einu sinni – once
í bíó – to the cinema

Meaning difference:

Við fórum einu sinni í bíó. – Simple past: a specific event in the past (“we went once”).
Við höfum einu sinni farið í bíó. – Present perfect: emphasizes life experience up to now (“we have been once in our lives”).

In everyday conversation, Icelanders often use the simple past where English might use the present perfect, so Við fórum einu sinni í bíó can sometimes also be translated as We’ve been to the cinema once, depending on context.

How would I say “We went to the cinema twice / three times” using the same pattern?

You can replace einu sinni with the appropriate adverb:

Við fórum tvisvar í bíó. – We went to the cinema twice.
Við fórum þrisvar í bíó. – We went to the cinema three times.

You can also use sinnum (“times”):

Við fórum tvisvar sinnum í bíó.
Við fórum þrisvar sinnum í bíó.

For 4 and above, you normally use a number in the dative plural + sinnum:

Við fórum fjórum sinnum í bíó. – four times
Við fórum tíu sinnum í bíó. – ten times

So your original einu sinni fits into a wider pattern of counting how many times something happened.