Um kvöldið byrjaði þrumuveður, og við sáum eldingar yfir húsunum.

Um kvöldið is a time expression meaning that evening or during the evening.

Icelandic often puts a time phrase first to set the scene. English can do the same:

• That evening, a thunderstorm began.
• During the evening, a thunderstorm began.

Starting with the time phrase also affects word order in the rest of the clause.

These expressions are similar, but they are not used in exactly the same way.

• í kvöld usually means tonight
• um kvöldið means that evening / during the evening

So um kvöldið refers to a specific evening in a narrative, while í kvöld usually refers to the coming or current evening from the speaker’s point of view.

Kvöldið is in the accusative singular definite form.

That is because the preposition um often takes the accusative, including in many time expressions.

Here:

• kvöld = evening
• kvöldið = the evening

Since kvöld is a neuter noun, the nominative and accusative singular definite look the same: kvöldið.

This is because Icelandic follows the V2 rule in main clauses. That means the finite verb usually comes in the second position.

The sentence begins with Um kvöldið, which takes the first position. So the verb byrjaði must come next:

• Um kvöldið = first position
• byrjaði = second position
• þrumuveður = subject

A more neutral order would be:

• Þrumuveður byrjaði um kvöldið.

Both are grammatical, but the original version puts emphasis on the time.

Byrjaði is the past tense of byrja = to begin / to start.

In this sentence it is 3rd person singular past, because the subject is þrumuveður, which is singular.

So:

• byrja = to begin
• byrjaði = began

Icelandic does not have an indefinite article like English a/an.

So þrumuveður can mean:

• thunderstorm
• a thunderstorm

If Icelandic wants to say the thunderstorm, it usually adds the definite article to the noun itself:

• þrumuveðrið = the thunderstorm

Yes. Icelandic very often makes compound nouns.

Here:

• þruma = thunder
• veður = weather

Together, þrumuveður means thunderstorm.

This is very normal in Icelandic, and you will see many similar compounds.

Sáum is the 1st person plural past tense of sjá = to see.

So:

• ég sá = I saw
• við sáum = we saw

This is an irregular verb, so the past tense does not look like the infinitive very closely.

Yes, Icelandic normally does include the subject pronoun in ordinary sentences.

So við sáum is the normal way to say we saw.

Because the verb ending already shows person and number, the pronoun can sometimes be omitted in special contexts, but in standard usage it is usually present.

In Icelandic, eldingar often refers to flashes of lightning or lightning bolts, so the plural is very natural.

Here the idea is that they saw multiple flashes in the sky, so eldingar fits well.

• elding = a lightning flash / a bolt of lightning
• eldingar = lightning flashes / bolts

English often uses uncountable lightning, but Icelandic frequently uses the countable plural.

The preposition yfir can take different cases depending on the meaning:

• accusative if there is movement toward/over something
• dative if it describes location or position

Here the lightning is being seen over the houses as a location, so Icelandic uses the dative:

• yfir húsunum = over the houses

If it were more about movement across or over them, accusative could be used in other contexts.

Húsunum is the dative plural definite form of hús = house.

So:

• hús = house / houses
• húsin = the houses
• húsunum = to/in/on/over the houses, depending on the preposition and context

The ending -unum is a very common dative plural definite ending for neuter nouns like hús.

That is just how the definite article usually works in Icelandic. Instead of a separate word like English the, Icelandic usually adds it as an ending.

For example:

• kvöld = evening

• kvöldið = the evening

• hús = house / houses
• húsunum = the houses, in the dative plural

So when you see endings like -ið, -in, or -unum, they often include the idea of the.

Not exactly word-for-word, but it can sound natural in English depending on context.

The Icelandic sentence literally says that a thunderstorm began. English might also say:

• That evening, a thunderstorm started
• That evening, a thunderstorm began
• sometimes more loosely, That evening, there was a thunderstorm

But the Icelandic verb byrjaði clearly expresses the idea of beginning, so started/began is the closest match.