Það var verið að þrífa íþróttahúsið þegar við komum, svo keppnin byrjaði aðeins seinna.

This is a very common Icelandic way to say that an action was in progress.

• það = a dummy subject, like English there/it in expressions such as there was...
• var verið að + infinitive = was in the process of ... / was going on

So:

• Það var verið að þrífa = Cleaning was going on / Someone was cleaning / They were cleaning

It is an impersonal construction, so it does not name who was doing the cleaning. English often uses they were cleaning or the gym was being cleaned, depending on context.

This pattern is very useful in Icelandic:

• Það var verið að elda = Cooking was going on
• Það er verið að laga bílinn = They are repairing the car / The car is being repaired

No, not really. In this sentence, það is a dummy pronoun. It does not point to a specific thing.

Its job is mainly grammatical: Icelandic often wants something in the subject position, and það can fill that slot in impersonal sentences.

So in:

• Það var verið að þrífa íþróttahúsið

the important idea is the action itself, not það.

Because vera að + infinitive and vera verið að + infinitive are not exactly the same.

• vera að + infinitive usually means a specific person or thing is doing something:

  • Ég var að þrífa = I was cleaning
  • Hann var að lesa = He was reading

• það var verið að + infinitive is more impersonal:

  • Það var verið að þrífa = Cleaning was going on / They were cleaning

So verið helps form this impersonal progressive-like expression. To an English speaker, was been cleaning might look odd, but that is not how it works in Icelandic. It is a fixed grammatical pattern.

Because it comes after að in the construction vera/vera verið að + infinitive.

So:

• að þrífa = to clean
• að lesa = to read
• að vinna = to work

In this sentence:

• var verið að þrífa = was in the process of cleaning

This is normal and expected after að in this pattern.

íþróttahúsið means the sports hall / the gymnasium.

It is made from:

• íþrótt = sport
• hús = house/building
• íþróttahús = sports hall
• íþróttahúsið = the sports hall

The ending -ið is the definite article attached to the noun. Icelandic usually adds the to the end of the noun rather than using a separate word.

So:

• íþróttahús = a sports hall
• íþróttahúsið = the sports hall

It is the direct object of þrífa, so it is in the accusative case.

However, for many neuter singular nouns, the nominative and accusative forms look the same. That is what happens here:

• nominative: íþróttahúsið
• accusative: íþróttahúsið

So even though the form does not change, its grammatical role is accusative.

komum is the 1st person plural past tense of koma:

• ég kom = I came
• við komum = we came

So:

• þegar við komum = when we arrived / when we came

This clause gives the time background for the first part of the sentence:

• Cleaning was going on when we arrived.

Most naturally here, it means when we came / when we arrived.

Icelandic often uses the simple past where English might sometimes choose a different wording depending on context. In this sentence, the idea is straightforward:

• the cleaning was happening
• then we arrived
• because of that, the competition started later

So when we arrived is usually the best natural English understanding.

Here svo means so, in the sense of as a result.

In this sentence:

• svo keppnin byrjaði aðeins seinna = so the competition started a little later

Depending on context, svo can also mean:

• then
• and then
• so

But here it clearly links cause and result:

• the sports hall was being cleaned,
• so the competition started later.

Because keppnin means the competition, while keppni means a competition / competition.

Again, Icelandic usually adds the definite article to the end of the noun:

• keppni = competition
• keppnin = the competition

So:

• keppnin byrjaði = the competition started

aðeins seinna means a little later or slightly later.

Breakdown:

• aðeins = a little, slightly, only
• seinna = later

So:

• byrjaði aðeins seinna = started a little later

The comparative seinna does not need an explicit than... phrase here. It simply means later than expected / later than usual / later than planned, depending on context.

Icelandic does follow a verb-second pattern in main clauses, but here the order is still normal for that structure.

In:

• svo keppnin byrjaði aðeins seinna

the first element is svo, and the finite verb is byrjaði. The subject keppnin comes before the verb here because svo is functioning like a coordinating link, and this clause is felt very much like a new main clause with ordinary subject-verb order.

You will often see Icelandic word order vary depending on the type of connector and the structure of the sentence, so this is something learners gradually get used to. The important thing for now is that this sentence is natural Icelandic.

Yes, absolutely. That is a very natural English translation.

Even though the Icelandic structure is literally more like There was being cleaning of the sports hall, the best English equivalent is often:

• The sports hall was being cleaned when we arrived, so the competition started a little later.

Another possible translation is:

• They were cleaning the sports hall when we arrived, so the competition started a bit later.

The Icelandic version leaves the agent unspecified, so both English translations can work depending on style.