Ef það verður aftur þrumuveður á næstu hátíð, vitum við allavega hvar skýlið er.

Ef means if. It introduces a conditional clause, so the first part of the sentence sets up a possible situation:

Ef það verður aftur þrumuveður á næstu hátíð
= If there is a thunderstorm again at the next festival

The rest of the sentence then gives the result or consequence.

Here það works like a dummy subject, similar to it or there in English.

In weather and event expressions, Icelandic often uses það even when it does not refer to a specific thing. So:

það verður þrumuveður
means something like there will be a thunderstorm or it will be stormy/thundery.

So það is there because Icelandic normally wants a subject in that kind of clause.

Verður is from verða, while er is from vera. Both can connect to the idea of to be, but they are not used in the same way.

In this sentence, verður is natural because it refers to something that will happen or arise:

það verður þrumuveður
= there will be a thunderstorm

Later in the sentence, er is used in a simple identifying/location-type statement:

skýlið er
= the shelter is

So the sentence uses two different Icelandic verbs that English often translates with forms of to be.

Icelandic often uses the present tense for future meaning, especially when the future is understood from the context.

That is happening here. The clause is about a future possibility, but Icelandic still says:

Ef það verður...
literally with a present-tense form.

This is actually similar to English. We normally say:

If it rains tomorrow...
not If it will rain tomorrow...

So this part is less strange than it may first seem.

Þrumuveður is a compound noun:

• þruma = thunder
• veður = weather

Together, þrumuveður means thunderstorm.

Compound nouns are very common in Icelandic, so learning to spot the parts can help a lot with vocabulary.

This means at the next festival.

A few things are going on here:

• á often means on, but with events and occasions it can often be understood more like at
• hátíð means festival, celebration, or holiday, depending on context
• næstu is the form of næstur that fits this noun phrase here

The important learner point is that adjectives in Icelandic change form to match the noun they describe, and prepositions often affect that form too. So næstu is not random; it is the grammatically correct form in this phrase.

This is because of Icelandic verb-second word order.

In a main clause, the finite verb usually comes in the second position. Since the sentence starts with the ef-clause, that whole clause takes the first slot. Then the main clause begins, and the verb comes next:

Ef ... , vitum við allavega ...

So after the comma, Icelandic puts vitum before við.

If the clause stood by itself, you could say:

Við vitum allavega hvar skýlið er.

But after an opening clause like this, inversion is expected.

Allavega means at least, anyway, or in any case, depending on context.

In this sentence, at least is the best fit:

vitum við allavega hvar skýlið er
= at least we know where the shelter is

It is a very common everyday word. In tone, it is fairly conversational and natural.

Because this is an embedded question, not a direct question.

Compare:

• Direct question: Hvar er skýlið? = Where is the shelter?
• Embedded question: Við vitum hvar skýlið er. = We know where the shelter is.

English works the same way:

• Where is the shelter?
• We know where the shelter is.

So once hvar is inside a larger sentence after vitum, Icelandic uses normal subordinate-clause word order: subject first, then verb.

Skýli means shelter. It is a neuter noun.

The ending -ið is the definite article attached to the noun, so:

• skýli = a shelter
• skýlið = the shelter

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

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

So:

Ef það verður aftur þrumuveður á næstu hátíð,
is the condition,

and

vitum við allavega hvar skýlið er.
is the main statement.

This kind of comma is normal and helpful in Icelandic, especially when a sentence begins with an if-clause.