Í júlí og ágúst er borgin oft rólegri af því að margir eru í fríi.

This is because Icelandic normally follows a verb-second pattern in main clauses.

That means the finite verb usually comes in the second position of the sentence, no matter what comes first.

So in:

Í júlí og ágúst er borgin oft rólegri ...

• Í júlí og ágúst = the first element
• er = the finite verb, so it comes second
• borgin = the subject, which comes after the verb here

This is very common in Icelandic. If you start with a time phrase, place, or another element, the verb still usually stays in second position.

Yes, here í works much like English in, and the whole phrase means in July and August or during July and August.

In Icelandic, í is often used with months:

• í janúar = in January
• í júlí = in July
• í ágúst = in August

So í júlí og ágúst means that something happens during those months.

Because borgin means the city, while borg means just city.

Icelandic usually adds the definite article to the end of the noun instead of using a separate word like English the.

So:

• borg = city
• borgin = the city

This attached -in is the definite article.

Oft means often, and its position here is very natural:

er borgin oft rólegri

In Icelandic, adverbs like oft, alltaf, aldrei, and stundum often appear after the verb and around the predicate part of the sentence.

So this word order is normal and idiomatic. English learners sometimes expect a stricter English-like placement, but Icelandic allows some flexibility. In this sentence, oft fits naturally between borgin and rólegri.

Rólegri is the comparative form of rólegur, which means calm, quiet, or peaceful.

So:

• rólegur = quiet / calm
• rólegri = quieter / calmer

Here it describes borgin, so the meaning is the city is often quieter.

A useful thing to notice is that Icelandic comparatives often end in -ri:

• stór → stærri = bigger
• gamall → eldri = older
• rólegur → rólegri = quieter

Because Icelandic, like English, can use a comparative without explicitly saying what it is being compared to.

So borgin er oft rólegri means something like:

• the city is often quieter
• implicitly: quieter than usual, quieter at that time, or quieter than in other months

If Icelandic wanted to state the comparison directly, it could use en = than. But here the comparison is left understood.

Af því að means because.

In this sentence:

... af því að margir eru í fríi = ... because many are on vacation

It is a very common way to introduce a reason.

You may also see related forms such as:

• því að = because
• af því = because of that / from that, depending on context

But in this sentence, af því að is simply a conjunction meaning because.

In Icelandic, margir can stand on its own and mean many people when the noun is understood from context.

So:

• margir eru í fríi = many are on vacation / many people are on vacation

This is very natural. English does the same thing:

• Many are away
• Many are working

So no extra noun is needed.

Margir is the plural masculine nominative form of margur, meaning many.

It is used here because it is the subject of the clause:

margir eru í fríi

When Icelandic refers to a mixed group of people, or people in general, the masculine plural is often used by default.

So here margir is best understood as many people.

Because after í, Icelandic often uses the dative when it means being in a state or location, rather than moving into it.

The noun is frí = vacation / holiday / time off, and here the dative form is fríi.

So:

• í fríi = on vacation / on holiday

This is a very common expression, and it is best learned as a set phrase.

Even though English says on vacation, Icelandic uses í here.

Because the verb vera = to be must agree with its subject.

In the main clause:

• borgin = the city → singular
• so the verb is er = is

In the subordinate clause:

• margir = many people → plural
• so the verb is eru = are

So:

• borgin er = the city is
• margir eru = many are / many people are

This is just normal subject-verb agreement.