Í júní eru kvöldin bjartari en í september.

This is because Icelandic is a verb-second language.

When a time phrase like Í júní (in June) is placed first, the finite verb usually comes next. So the order becomes:

Í júní + eru + kvöldin + bjartari ...

A more neutral order is also possible:

Kvöldin eru bjartari í júní en í september.

Both are correct, but the original sentence puts a little more focus on Í júní.

Because kvöldin is plural.

• er = is
• eru = are

Since the subject is kvöldin (the evenings), the verb must be plural: eru.

Kvöldin means the evenings.

The basic noun is kvöld, which means evening. In this sentence it refers to evenings in general during June and September.

The ending -in is the definite article attached to the noun.

So:

• kvöld = evenings / evening
• kvöldin = the evenings

In Icelandic, the is very often added to the end of the noun instead of being a separate word.

Because the sentence is talking about evenings in general across a whole month, not about one single evening.

So Icelandic uses the plural kvöldin: the evenings.

English does the same naturally here: Evenings are brighter in June than in September or The evenings are brighter...

Yes. Bjartari means brighter.

It is the comparative form of bjartur, which means bright.

So:

• bjartur = bright
• bjartari = brighter

Icelandic usually forms this kind of comparison by changing the adjective itself, rather than adding a separate word like more.

Because the sentence is making a comparison:

June evenings are brighter than September evenings.

Whenever you compare two things, Icelandic uses the comparative form of the adjective. So bjartur becomes bjartari.

En means than in this sentence.

It connects the two parts of the comparison:

• bjartari en ... = brighter than ...

Be careful: en can also mean but in other contexts, but here it clearly means than.

Because í is the preposition used for in or during with months.

So:

• í júní = in June
• í september = in September

That is the normal way to express time with months in Icelandic.

Usually kvöld means evening.

If you want night, the usual word is nótt.

So in this sentence, kvöldin bjartari means the evenings are brighter/lighter, which makes sense when talking about seasonal daylight.

Yes, that is also correct.

It is a little more straightforward in word order:

Kvöldin eru bjartari í júní en í september.

The original version,

Í júní eru kvöldin bjartari en í september,

simply brings Í júní to the front for emphasis or topic. Icelandic does this very often.