Þegar þrumuveðrið var mjög slæmt, heyrðum við þrumu og sáum fólk hlaupa í skýlið.

Þegar means when. It introduces a time clause: Þegar þrumuveðrið var mjög slæmt = When the thunderstorm was very bad/severe.

This kind of clause sets the scene for the main action. After that clause, Icelandic normally puts the finite verb of the main clause next, which is why you get heyrðum við rather than við heyrðum.

This is because of the Icelandic V2 rule: in main clauses, the finite verb usually comes in the second position.

Here, the whole clause Þegar þrumuveðrið var mjög slæmt takes the first position. So the finite verb of the main clause, heyrðum, comes next, and then the subject við follows it:

• Þegar þrumuveðrið var mjög slæmt, heyrðum við ...

If there were no opening þegar clause, you would normally say:

• Við heyrðum þrumu og sáum fólk hlaupa í skýlið.

Þrumuveður means thunderstorm, while þrumuveðrið means the thunderstorm.

The ending -ið is the suffixed definite article for a neuter singular noun. Icelandic often attaches the to the end of the noun instead of using a separate word.

So:

• þrumuveður = a thunderstorm / thunderstorm
• þrumuveðrið = the thunderstorm

Because slæmt has to agree with þrumuveðrið.

Þrumuveðrið is:

• singular
• neuter
• nominative here

So the adjective also appears in the neuter singular form:

• slæmur = masculine
• slæm = feminine
• slæmt = neuter

Since þrumuveðrið is neuter, slæmt is the correct form.

It is in the nominative case.

In the clause þrumuveðrið var mjög slæmt, þrumuveðrið is the subject of var. Subjects are normally in the nominative.

Also, after vera (to be), the predicate adjective agrees with the subject, which is why you get slæmt.

Because þrumu is the accusative singular form of þruma.

The verb heyra (to hear) takes a direct object in the accusative. So:

• nominative: þruma
• accusative: þrumu

That is why the sentence says:

• heyrðum við þrumu = we heard thunder

They are all past tense forms.

• var = was
• heyrðum = heard
• sáum = saw

So the whole sentence is describing past events.

Fólk is the direct object of sáum, so it is in the accusative. But the important thing is that fólk has the same form in nominative and accusative singular, so you do not see a change here.

So even though the case changes grammatically, the word itself stays fólk.

After verbs of perception like sjá (to see) and heyra (to hear), Icelandic often uses a construction with an object plus a bare infinitive.

So:

• sáum fólk hlaupa = saw people run/running

This is very natural in Icelandic. The infinitive hlaupa describes what the people were seen doing.

Hlaupa is the infinitive of to run.

In sáum fólk hlaupa, it works with fólk after the perception verb sáum:

• sáum = saw
• fólk = people
• hlaupa = run / running

A natural English translation is often saw people running, even though Icelandic uses the infinitive form here.

Because í can take either:

• accusative for motion into
• dative for location in

Here there is movement: people are running into the shelter, not simply being in the shelter. So Icelandic uses the accusative:

• í skýlið = into the shelter

If it meant location, it would be:

• í skýlinu = in the shelter

Both come from skýli (shelter, a neuter noun), but they show different cases and meanings:

• skýlið = accusative/nominative definite singular, often used after motion with í
• skýlinu = dative definite singular, often used for location with í

So:

• hlaupa í skýlið = run into the shelter
• vera í skýlinu = be in the shelter

Yes. Mjög means very, and slæmt means bad in the neuter singular form.

So:

• mjög slæmt = very bad

With weather, English might often prefer severe, and Icelandic can also use other words depending on style, but mjög slæmt is perfectly understandable and natural.

It has two main parts:

1. A subordinate time clause:

   • Þegar þrumuveðrið var mjög slæmt
   • When the thunderstorm was very bad

2. The main clause:

   • heyrðum við þrumu og sáum fólk hlaupa í skýlið
   • we heard thunder and saw people run into the shelter

Inside the main clause, there are two coordinated verbs:

• heyrðum
  • object þrumu
• sáum
  • object/complement fólk hlaupa í skýlið

So the sentence is built very systematically, even if the word order looks a little different from English.