Í maí og júní verður loftið hlýrra og kvöldin bjartari.

Because that phrase sets the time frame first: in May and June.

In Icelandic, it is very common to begin a sentence with a time expression like this. The preposition í here corresponds to English in.

You can think of Í maí og júní as the sentence’s when? part.

It is also possible to repeat the preposition and say í maí og í júní, but when the same preposition applies to both nouns, Icelandic often uses it just once.

This is because of Icelandic V2 word order in main clauses.

In a main clause, the finite verb usually comes in the second position. Since Í maí og júní takes the first position, verður must come next:

• Í maí og júní = first slot
• verður = second slot
• loftið = after the verb

So Icelandic prefers:

• Í maí og júní verður loftið hlýrra ...

rather than:

• Í maí og júní loftið verður ...

This is one of the most important word-order patterns in Icelandic.

Because verða means become / get, while vera means be.

So:

• loftið verður hlýrra = the air becomes / gets warmer
• loftið er hlýrra = the air is warmer

In this sentence, the idea is that conditions are changing as the season moves into May and June, so verður is the natural choice.

Because the second verb is understood and left out.

The full version would be:

• Í maí og júní verður loftið hlýrra og kvöldin verða bjartari.

In other words:

• loftið verður hlýrra
• kvöldin verða bjartari

Icelandic often omits a repeated verb when it is easy to understand from context. English does this too sometimes, as in The air gets warmer and the evenings brighter, though English often prefers repeating the verb less than Icelandic does.

They show the definite article: the.

In Icelandic, the is usually attached to the end of the noun rather than written as a separate word.

So:

• loft = air
• loftið = the air

and

• kvöld = evening
• kvöldin = the evenings

This is a very common Icelandic pattern.

Because the sentence is talking about evenings in general during May and June, not one specific evening.

So:

• kvöldin bjartari = the evenings brighter

This refers to the repeated pattern across those months: the evenings as a group become brighter.

If you said kvöldið, that would mean the evening singular, which would sound like you were talking about one particular evening.

They are in the nominative, because they are the subjects of the verb.

Even though the second verb is omitted, the structure is still:

• loftið verður ...
• kvöldin verða ...

Both loftið and kvöldin are the things that become warmer/brighter, so they are subjects, and subjects are normally nominative in Icelandic.

They are comparative adjectives:

• hlýrra = warmer
• bjartari = brighter

In Icelandic, adjectives used after vera or verða are still real adjectives, and they agree with the noun they describe.

Here:

• loftið is neuter singular, so the adjective appears as hlýrra
• kvöldin is plural (neuter plural), so the adjective appears as bjartari

So even though these adjectives come after the noun and verb, they still match the noun grammatically.

Because two different things are happening:

1. They come from different adjectives

   • hlýr = warm
   • bjartur = bright

2. They must agree with different nouns

   • loftið = neuter singular
   • kvöldin = plural

Also, hlýr has a less straightforward comparative pattern than a regular adjective like bjartur.

A useful way to learn this sentence is:

• hlýr → hlýrri / hlýrra
• bjartur → bjartari

So the difference is not random; it comes from both adjective formation and agreement.

No. It is still an adjective.

In this sentence, hlýrra and bjartari are predicate adjectives. That means they come after a verb like vera or verða and describe the subject:

• loftið verður hlýrra = the air becomes warmer
• kvöldin verða bjartari = the evenings become brighter

English works similarly:

• The air becomes warmer
• The evenings become brighter

So the position after the verb does not make them adverbs.

Here, loftið means the air or the atmosphere.

That is the natural meaning in a weather or seasonal sentence.

The noun loft can have other related meanings depending on context, but in this sentence it clearly refers to the air outside, not anything like a ceiling or a room interior.

Yes, to some extent.

For example, you could also say:

• Loftið verður hlýrra og kvöldin bjartari í maí og júní.

That is grammatical too. The main difference is focus:

• Í maí og júní ... puts the time first
• Loftið verður hlýrra ... puts the statement about the air first

But in a normal main clause, Icelandic still follows the verb-second rule, so whichever element comes first, the finite verb usually comes next.