Sumarið er hlýtt í borginni.

to be

Breakdown of Sumarið er hlýtt í borginni.

• í: in
• sumarið: the summer
• hlýtt: warm
• borginni: the city

Questions & Answers about Sumarið er hlýtt í borginni.

Why does sumarið end with -ið?

In Icelandic, definiteness (“the”) is marked by a suffix attached to the noun rather than a separate word.

• Sumar is the indefinite form (“summer,” a concept)
• Sumarið is the definite form (“the summer”)
  Here sumarið is nominative singular neuter, so it takes -ið to mean the summer.

Why is the adjective hlýtt spelled with -tt at the end instead of just -t?

Adjectives must agree with the noun they modify in gender, number, and case.

• Sumarið is neuter singular nominative.
• The strong-declension adjective for warm is hlýr (masculine), hlý (feminine), hlýtt (neuter).
  Thus in neuter nominative singular the correct ending is -tt, giving hlýtt.

Why is í borginni in the dative case?

The preposition í can trigger two cases:

• Accusative when expressing motion into something
• Dative when expressing static location in something
  Since we are talking about being in the city (location, not movement), í takes the dative.
  Borg (city) in definite dative singular becomes borg–+inni → borginni.

Could I use í borgina instead of í borginni?

No – borgina is the accusative definite form (“into the city”). You’d use that if you were moving into the city. For a stationary situation (“in the city”), you must use the dative borginni.

Why isn’t there a separate word for “the” like in English?

Icelandic has no standalone definite article. Instead, it attaches endings to nouns (and adjectives) to show definiteness. Those endings change with gender, number, and case, so you see different forms like -inn, -ið, -in, -num, etc.

Is Sumarið er hlýtt í borginni the most natural way to say “The summer is warm in the city”?

It’s perfectly grammatical and emphasizes sumarið at the beginning. However, a more common everyday phrasing is:
Það er hlýtt í borginni um sumarið.
That literally means “It is warm in the city during the summer,” which sounds more idiomatic in spoken Icelandic.

Could I also say Á sumrin er hlýtt í borginni?

Yes. Á sumrin means “in the summers” or “during summer.” This shifts from speaking about this specific summer to summer seasons in general. Word order is flexible, so you get:
Á sumrin er hlýtt í borginni – “In summer it’s warm in the city.”