Þegar fyrsta þruman heyrist, hlaupa sumir inn í skýlið við hliðina á salnum.

Þegar means when and introduces a time clause:

• Þegar fyrsta þruman heyrist = when the first thunder/clap of thunder is heard

That whole clause sets the time for the main action. After it comes the main clause:

• hlaupa sumir inn í skýlið ... = some run into the shelter ...

So the sentence structure is:

• [time clause] + [main clause]

The comma marks the end of the opening clause.

This is a very common Icelandic word-order pattern. Icelandic is a verb-second language in main clauses.

When the sentence begins with something other than the subject — here, the whole þegar clause — the finite verb usually comes next:

• Þegar fyrsta þruman heyrist, hlaupa sumir ...

So:

1. Þegar fyrsta þruman heyrist = first position
2. hlaupa = verb in second position
3. sumir = subject after the verb

If the sentence started directly with the subject, you would get:

• Sumir hlaupa inn í skýlið ...

Both are grammatical, but the version in your sentence follows normal V2 word order after an opening clause.

Heyrist comes from the verb heyra = to hear.

The -st form is very common in Icelandic and often gives a middle/passive-like meaning. Here:

• heyra = hear
• heyrist = is heard / can be heard

So:

• fyrsta þruman heyrist = the first thunder is heard

Grammatically, heyrist is 3rd person singular present, agreeing with fyrsta þruman, which is singular.

Because the adjective has to agree with the noun in gender, number, case, and definiteness.

Here the noun is:

• þruma = a thunderclap / thunder
• feminine
• singular
• nominative

But in the sentence it is definite:

• þruman = the thunder / the thunderclap

When a noun is definite like that, the adjective usually takes the weak form:

• fyrsta þruman = the first thunderclap

So:

• fyrsti would be masculine nominative singular
• fyrsta is the correct weak form here with a feminine definite noun

Þruma is a feminine noun meaning thunder or a thunderclap / rumble of thunder, depending on context.

In this sentence, fyrsta þruman most naturally means something like:

• the first clap of thunder
• the first rumble of thunder

The -n at the end is the attached definite article, so:

• þruma = thunder / a thunderclap
• þruman = the thunder / the thunderclap

Sumir means some or some people here.

In Icelandic, words like sumir can stand on their own without a noun, just like English some in a sentence like Some run inside.

So:

• sumir = some, some people

It is plural, which is why the verb is also plural:

• hlaupa = run (plural)
• compare singular hleypur = runs

This combination is very common in Icelandic for movement into something.

• í = in / into
• inn adds the idea of motion inward

So:

• inn í skýlið = into the shelter

You will often see this pattern with motion:

• fara inn í hús = go into a house
• hlaupa inn í skýlið = run into the shelter

Without motion, Icelandic often just uses location:

• vera í skýlinu = be in the shelter

Skýli is a neuter noun meaning shelter, shed, or cover, depending on context.

Here it appears as skýlið, which is the definite singular form:

• skýli = a shelter
• skýlið = the shelter

Also, after inn í with motion toward something, í takes the accusative, and for this noun the definite accusative singular looks the same as the nominative:

• skýlið

So the form works both as the shelter and as the correct case after motion into it.

Við hliðina á is a fixed expression meaning:

• next to
• beside
• at the side of

So:

• við hliðina á salnum = next to the hall

It is best learned as a whole phrase, because the parts do not map perfectly word-for-word into natural English.

Because við hliðina á requires the noun after á to be in the dative.

The base noun is:

• salur = hall, room

Its definite singular dative form is:

• salnum = the hall / the room in dative

So:

• við hliðina á salnum = next to the hall

This is a good example of why Icelandic case matters: the prepositional phrase determines the form of the noun.

Icelandic often uses the present tense for:

• general truths
• repeated situations
• vivid narration

So this sentence can describe a typical reaction:

• When the first thunder is heard, some run into the shelter next to the hall.

That sounds natural in Icelandic. English can do something similar in general statements, even if in a specific story English might sometimes choose a past tense instead.