Einn strengur á fiðlunni var laus áður en hún byrjaði að spila.

Because strengur is a masculine singular nominative noun, and einn must agree with it.

In Icelandic, words like einn change form for gender, number, and case:

• einn = masculine
• ein = feminine
• eitt = neuter

So:

• einn strengur = one string
• ein fiðla = one violin
• eitt barn = one child

Here strengur is the subject of the sentence, so it is in the nominative case, and that gives einn strengur.

Because the preposition á here takes the dative when it means static location, and fiðlunni is the dative singular definite form of fiðla.

Breakdown:

• fiðla = violin
• fiðlan = the violin, nominative singular
• fiðlunni = the violin, dative singular

In á fiðlunni, the meaning is basically on the violin or idiomatically on the violin in the sense of belonging to it.

A useful rule:

• á + dative = location/state, on
• á + accusative = movement onto something

Here the string already is on the violin, so it is a location/state idea, which is why you get á fiðlunni.

This is just an idiomatic difference between Icelandic and English.

Icelandic naturally says:

• strengur á fiðlu / á fiðlunni = a string on a violin / on the violin

English often prefers:

• a violin string
• a string of the violin
• a string on the violin

So á fiðlunni is perfectly natural Icelandic even if English might phrase it differently.

Laus literally means loose, not fastened tightly, or not attached firmly. In this sentence, it means the string was loose.

That could suggest:

• it was not tightened properly
• it had slack
• it was not ready for playing

It does not necessarily mean broken. A broken string would usually be expressed more directly with another word or phrase.

Also, laus changes form to agree with the noun it describes. Since strengur is masculine singular, the adjective is laus.

Compare:

• laus strengur = a loose string
• laus fiðla would be wrong here because fiðla is feminine; that would need laus in a feminine form in the right context
• laust barn = neuter singular form

Because laus is a predicate adjective, and predicate adjectives agree with the subject in gender and number.

The subject is einn strengur, which is:

• masculine
• singular
• nominative

So the adjective must also be masculine singular:

• strengur var laus = the string was loose

If the subject were neuter, you would get a different form:

• band var laust = the ribbon/string-like thing was loose

So var laus is correct because it matches strengur.

Áður en means before and introduces a subordinate clause.

So:

• áður en hún byrjaði að spila = before she started to play

This is a very common Icelandic pattern:

• áður en ég fer = before I go
• áður en við borðum = before we eat

It works much like English before followed by a clause.

Because this is a subordinate clause, not a main clause.

Icelandic main clauses often follow the verb-second pattern, but subordinate clauses usually do not. After áður en, the clause follows a more regular subject-verb order:

• hún byrjaði að spila

That is completely normal.

So the structure is:

• Main clause: Einn strengur á fiðlunni var laus
• Subordinate clause: áður en hún byrjaði að spila

Because byrja is commonly followed by að + infinitive.

So:

• byrja að spila = start to play
• byrja að lesa = start to read
• byrja að vinna = start to work

This is very similar to English start to ....

So in the sentence:

• hún byrjaði að spila = she started to play

Yes, spila is commonly used for playing a musical instrument, and also for certain kinds of playing more generally, depending on context.

Examples:

• spila á fiðlu = play the violin
• spila á píanó = play the piano
• spila fótbolta = play football/soccer

In this sentence, because we already have fiðlunni earlier, að spila is naturally understood as to play the violin.

Grammatically, hún is feminine singular, so a learner may wonder about that. But in normal interpretation here, hún refers to a female person, not the violin.

Why?

Because the clause says:

• hún byrjaði að spila = she started to play

A violin does not normally start playing by itself, so the meaning strongly points to a woman or girl who is playing the instrument.

So even though fiðla is a feminine noun, context makes the intended reference clear.

No, Icelandic normally requires an explicit subject in a sentence like this.

So you say:

• hún byrjaði að spila

not just:

• byrjaði að spila

unless the sentence structure is different for some special reason.

English speakers usually find this familiar, because English also normally requires a subject.

Yes. The sentence is talking about one specific string, so singular is natural in both languages.

• Einn strengur = one string

If it were more than one, Icelandic would change both the number word and the noun form, for example:

• Tveir strengir = two strings

So singular here is straightforward and works much like English.

Here are the main dictionary forms:

• einn = one
• strengur = string
• á = on
• fiðla = violin
• vera = to be → here var
• laus = loose
• áður en = before
• hún = she
• byrja = begin, start
• að = to, infinitive marker
• spila = play

This can help because many of the forms in the sentence are inflected:

• fiðlunni comes from fiðla
• var comes from vera
• byrjaði comes from byrja

Only the pronoun would need to change:

• Einn strengur á fiðlunni var laus áður en hann byrjaði að spila.

That means One string on the violin was loose before he started to play.

So:

• hún = she
• hann = he

Everything else can stay the same.

The letter ð in Icelandic is usually like the th in English this, not like the th in thing.

So áður is approximately pronounced with that voiced th sound.

A rough learner-friendly guide:

• áður ≈ OW-thur or AU-thur, though not exactly like English

Also note:

• Icelandic pronunciation is often not obvious from spelling
• ð never appears at the beginning of native Icelandic words

So áður is a good word for getting used to that sound.

It is natural Icelandic.

The structure:

• Einn strengur á fiðlunni var laus
• áður en hún byrjaði að spila

is normal and idiomatic.

A native speaker might still choose slightly different wording depending on context, but this sentence is grammatically natural and not an awkward literal translation.