Um leið og annað lagið byrjar, ganga fleiri inn á tónleikana.

Yes, um leið og is a very common fixed expression. It means as soon as, just as, or at the same time that.

In this sentence:

• Um leið og annað lagið byrjar = As soon as the second song starts / Just as the second song starts

You do not usually translate it word-for-word. Literally, um leið is something like at the same moment or along the way, but together with og it functions as a time expression introducing a clause.

Here annað lagið most naturally means the second song.

A few useful points:

• annar / annað / önnur can mean other, another, or second, depending on context.
• Because lagið is definite (the song), annað lagið is understood as the second song rather than just another song.

So:

• lag = song
• lagið = the song
• annað lagið = the second song

This is one of those places where Icelandic uses annar in a way that English learners may find broader than English other.

The ending -ið is the definite article attached to the noun.

• lag = song
• lagið = the song

Icelandic usually adds the onto the end of the noun instead of using a separate word like English the.

Here lagið is:

• neuter singular
• nominative
• definite

So annað lagið literally looks like second song-the, but naturally means the second song.

Annað is the neuter form of annar.

Since lag is a neuter noun, the adjective/pronoun connected to it must also be neuter.

Forms of annar include:

• masculine: annar
• feminine: önnur
• neuter: annað

Because lag is neuter, Icelandic uses:

• annað lag

and with the definite noun:

• annað lagið

This is grammatical agreement: the modifying word matches the noun in gender, number, and case.

Because the subject is singular: annað lagið.

• annað lagið = the second song
• byrjar = starts

So the verb agrees with a singular subject.

Compare:

• lagið byrjar = the song starts
• lögin byrja = the songs start

This is because of Icelandic verb-second word order.

The sentence begins with a subordinate clause:

• Um leið og annað lagið byrjar = As soon as the second song starts

After that opening element, the finite verb of the main clause normally comes next:

• ganga = finite verb
• fleiri = subject

So Icelandic says:

• Um leið og ... , ganga fleiri inn ...

not:

• Um leið og ... , fleiri ganga inn ...

This is very normal in Icelandic. In a simple main clause with no fronted element, you could say:

• Fleiri ganga inn á tónleikana. = More people go in to the concert.

But once the time clause comes first, the verb moves into second position.

Fleiri means more or more people here.

It is being used on its own, without a noun being stated. Icelandic often does this when the missing noun is obvious from context.

So:

• fleiri = more, understood as more people

The full idea is something like:

• fleiri [gestir / áhorfendur / manns] ganga inn
• more [guests / audience members / people] come in

In natural English, more people come in is the best translation.

Yes, ganga literally means walk, but it is also often used in a broader sense for go or come on foot, especially when entering somewhere.

So in this sentence:

• ganga inn = go in, come in, enter

It does not have to emphasize the act of walking in a very literal way. It is just a natural verb for people entering a place.

You may also see other verbs in similar contexts, such as:

• fara inn = go in
• koma inn = come in

But ganga inn sounds very natural for people physically entering.

Here they work together:

• inn = in / inside
• á = a preposition that can mean on, to, or sometimes at, depending on context
• inn á = in to, into, or inside to

In this sentence:

• ganga inn á tónleikana

means something like:

• go in to the concert
• enter the concert

This sounds a bit strange if translated word-for-word into English, but it is idiomatic in Icelandic.

The important idea is movement inward toward the event/venue.

Because this sentence describes movement toward/into something, so Icelandic uses the accusative after the preposition á.

• á tónleikana = to the concert(s) / into the concert
• á tónleikunum = at the concert(s)

This is a very important pattern in Icelandic:

• motion toward a place/event → often accusative
• location in/at a place/event → often dative

So compare:

• Þau fara á tónleikana. = They go to the concert.
• Þau eru á tónleikunum. = They are at the concert.

In your sentence, people are entering, so movement is involved, and á tónleikana is correct.

The Icelandic noun tónleikar is normally a plural noun, even when English would often use singular concert.

So:

• tónleikar = concert
• tónleikarnir = the concert
• tónleikana = accusative form of the concert

This is similar to how some nouns in one language may be singular while the natural equivalent in another language is plural.

So although the form is plural in Icelandic, the English meaning can still be singular:

• á tónleikana = to the concert

It is because of á in a movement sense.

The word inn adds the idea of inward motion, but the case is controlled by the prepositional phrase á tónleikana.

With á:

• accusative often indicates motion to/toward
• dative often indicates being at/on/in a place or event

So the key contrast is:

• inn á tónleikana = in to the concert / into the concert area
• á tónleikunum = at the concert

Not very naturally in English. A better translation would be:

• As soon as the second song starts, more people come in.
• As soon as the second song starts, more people enter the concert hall/venue.

The Icelandic phrase ganga inn á tónleikana refers to people entering the concert or the place where it is happening. English usually prefers:

• come in
• go in
• enter the venue / hall

rather than enter the concert, unless the context is very loose.

The comma separates the opening subordinate clause from the main clause.

Structure:

• Um leið og annað lagið byrjar = subordinate time clause
• ganga fleiri inn á tónleikana = main clause

So the comma helps show the boundary between the two parts.

In Icelandic, commas are often used before or after subordinate clauses more regularly than in everyday English.

Yes. That would also be grammatical.

• Þegar = when
• Um leið og = as soon as / just as

The difference is nuance:

• Þegar annað lagið byrjar... = When the second song starts...
• Um leið og annað lagið byrjar... = As soon as / just as the second song starts...

So um leið og gives a stronger sense of immediate timing.

It is:

1. subordinate time clause
2. main clause with verb-second order

Broken down:

• Um leið og = conjunction phrase introducing a time clause
• annað lagið = subject of the subordinate clause
• byrjar = verb of the subordinate clause
• ganga = finite verb of the main clause
• fleiri = subject of the main clause
• inn á tónleikana = direction/destination phrase

So literally the grammar is roughly:

• As soon as the second song starts, come in more people into the concert

But natural English is:

• As soon as the second song starts, more people come in.

Yes. In the main clause, fleiri is the subject, even though it comes after ganga.

That happens because of verb-second word order. Icelandic does not rely only on word order the way English does.

So in:

• ganga fleiri inn á tónleikana

the roles are:

• ganga = verb
• fleiri = subject

English learners often expect the subject to come first, but Icelandic often puts the finite verb before the subject after an opening phrase or clause.

Yes. Then the word order becomes more straightforward:

• Fleiri ganga inn á tónleikana um leið og annað lagið byrjar.

This is grammatical, but it places the time information later. The original version is more natural if you want to emphasize the timing first:

• Um leið og annað lagið byrjar, ganga fleiri inn á tónleikana.

That version highlights exactly when more people come in.