Sjórinn er rólegur í dag.

Because Icelandic usually puts the definite article onto the end of the noun.

• sjór = sea
• sjórinn = the sea

So -inn here is the masculine singular definite article. This is very common in Icelandic.

Because the adjective has to agree with the noun it describes.

Here, sjórinn is:

• masculine
• singular
• nominative

So the adjective also has to be masculine singular nominative:

• rólegur

Compare:

• masculine: rólegur
• feminine: róleg
• neuter: rólegt

Since sjór is a masculine noun, rólegur is the correct form.

Because rólegur is not directly attached to the noun inside the noun phrase here. It is a predicate adjective, used after the verb vera (to be).

So the structure is:

• Sjórinn = the subject
• er = is
• rólegur = calm

This works like English The sea is calm.

If you wanted the adjective directly before the noun, you would say something like:

• rólegur sjór = a calm sea

That is a different structure.

Er is the present tense, 3rd person singular form of vera (to be).

So:

• ég er = I am
• þú ert = you are
• hann / hún / það er = he / she / it is

In this sentence, sjórinn is singular, so er is the correct form.

Because sjórinn is the subject of the sentence, and subjects are normally in the nominative.

Also, with vera (to be), the adjective describing the subject is usually also in the nominative, agreeing with it.

So:

• sjórinn = nominative subject
• rólegur = nominative adjective agreeing with the subject

This is similar to how Icelandic often treats predicate adjectives after vera.

Í dag is a fixed expression meaning today.

Literally, it comes from:

• í = in
• dag = day

But as a whole, í dag simply means today.

It functions as a time expression in the sentence.

Yes. Icelandic word order is fairly flexible, especially with time expressions.

These are all natural or possible depending on context:

• Sjórinn er rólegur í dag.
• Í dag er sjórinn rólegur.

Both mean the same basic thing, but starting with Í dag gives more emphasis to today.

You would use the adjective directly with the noun:

• rólegur sjór = a calm sea

That is different from:

• Sjórinn er rólegur = the sea is calm

So:

• rólegur sjór = adjective + noun
• sjórinn er rólegur = noun + is
  • adjective

If you wanted the calm sea, you would usually say:

• rólegi sjórinn

There the adjective and noun are both definite.

A rough guide for English speakers:

• sj is usually pronounced a bit like sy
• ó is like a long o-type vowel
• -inn is pronounced with a short final syllable

A rough approximation is:

• SYO-rin

But Icelandic pronunciation is not exactly the same as English sounds, so it is best to listen to native audio if possible.

Also, the accented vowel ó is important: it is not the same as plain o.

The accent marks in Icelandic are part of the spelling and usually show a different vowel sound, not just stress.

Here:

• ó is a different vowel from o
• á would also be different from a, and so on

So you should treat accented vowels as important spelling and pronunciation differences, not as optional marks.

Usually sjór means sea, but in real use it can sometimes refer more generally to the sea as a body of salt water, and context may make English ocean possible in translation.

In this sentence, the sea is the most natural understanding.

It describes a current condition, especially because of í dag (today).

So the sentence means the sea is calm today, not necessarily that the sea is always calm.

Without í dag, the sentence would sound more general:

• Sjórinn er rólegur. = The sea is calm.

With í dag, it is clearly tied to today’s situation.