Hún er róleg í dag.

Word by word:

• Hún = she
• er = is (3rd person singular of vera = to be)
• róleg = calm, quiet, relaxed (feminine form of the adjective rólegur)
• í = in (here used in a time expression)
• dag = day (the word dagur “day” in the accusative singular, without an article)

So literally: “She is calm in day”, but idiomatically: “She is calm today.”

Icelandic adjectives agree with the noun (or pronoun) they describe in:

• Gender (masculine / feminine / neuter)
• Number (singular / plural)
• Case (nominative / accusative / dative / genitive)

Here, the adjective róleg- is describing hún (she), which is:

• Feminine
• Singular
• Nominative (subject of the sentence)

The weak nominative singular forms of rólegur are:

• Masculine: rólegi
• Feminine: rólega

But in a predicative position after vera (to be), Icelandic normally uses the strong adjective endings:

• Masculine: rólegur
• Feminine: róleg
• Neuter: rólegt

So:

• Hún er róleg. – She is calm. (feminine, strong form)
• Hann er rólegur. – He is calm. (masculine)
• Barnið er rólegt. – The child is calm. (neuter)

That’s why you see róleg, not rólegur: it’s agreeing with feminine hún and using the strong form.

Róleg(ur) covers several nuances, depending on context:

• Calm, relaxed, not stressed:
  
  • Hún er róleg í dag. – She is calm/relaxed today.
• Quiet, not noisy or agitated:
  
  • Krakkarnir eru óvenju rólegir. – The kids are unusually quiet/calm.

In this sentence, Hún er róleg í dag, it usually means:

• She’s calm / relaxed today,
• Possibly also quieter than usual, not nervous or upset.

It does not normally mean “boring” or “emotionless”; just not tense, excited, or noisy.

The phrase í dag (today) makes it clearly temporary:

• Hún er róleg. – She is calm.
  
  • Could mean either a general characteristic (She’s a calm person) or her current state, depending on context.
• Hún er róleg í dag. – She is calm today.
  
  • Strongly suggests this is about today’s state, possibly in contrast to other days.

So the sentence is understood as: “Today, she’s calm (even if maybe she isn’t always).”

Í dag is a very common fixed expression for “today.”

• í literally means “in”, but with time words it often means “on / during / this”:
  
  • í dag – today
  • í kvöld – this evening, tonight
  • í vetur – this winter

You do not use the article here. So:

• Correct:
  
  • Hún er róleg í dag. – She is calm today.
• Incorrect / unnatural for “today”:
  
  • ✗ Hún er róleg á dag.
  • ✗ Hún er róleg í deginum.

Í deginum could exist in some very specific, more literal contexts (“in the day” as opposed to “in the night”), but for “today” you must use í dag.

The base form of day is:

• dagur – day (nominative singular)

The full declension (indefinite) is:

• Nominative: dagur
• Accusative: dag
• Dative: degi
• Genitive: dags

In í dag, the noun is in the accusative singular (dag).

Preposition í can take accusative or dative depending on meaning:

• Í + accusative: movement into, or often a specific time span
  
  • í dag – today
• Í + dative: location in something (no movement)
  
  • í skóla (dat.) – in school

So we get í dag (not í dagur).

Yes, that’s perfectly correct:

• Hún er róleg í dag.
• Í dag er hún róleg.

The meaning is basically the same (“She is calm today”), but the emphasis changes slightly:

• Hún er róleg í dag.
  
  • Neutral; light emphasis on róleg í dag together.
• Í dag er hún róleg.
  
  • Emphasizes today more, a bit like “Today, she’s calm.”, often implying contrast:
    • She might be different on other days.

Both are very natural; the second just foregrounds “today”.

You insert ekki (not) after the verb er:

• Hún er ekki róleg í dag.
  
  • She is not calm today.

General pattern:

• [Subject] + er + ekki + [adjective] (+ time phrase)
  
  • Hann er ekki þreyttur. – He is not tired.
  • Þau eru ekki tilbúin enn. – They are not ready yet.

So here: Hún er ekki róleg í dag.

A fairly close IPA transcription (careful speaker) would be:

• [huːn ɛr ˈrouːlɛɣ iː daːɣ]

Approximate English-style guidance:

• Hún → “hoon” with a long oo (like food), but lips rounded: hoo-n
• er → like English air but short and clear: ehr
• róleg → ROH-leg
  
  • ró: long “roh” (as in row),
  • leg: “leg” with a soft final g (often like a soft fricative, almost like clearing your throat very gently).
• í → like a long ee in see: ee
• dag → daag, with a long a (somewhere between father and dog, depending on your English), and the final g also soft or weakened.

In everyday speech, the final g/ɣ sounds in róleg and dag may be quite light or almost disappear.

You need a plural subject pronoun and a plural adjective. Which pronoun you choose depends on the gender of “they”:

• Group of women only:
  
  • Þær eru rólegar í dag.
    
    • þær – they (feminine)
    • rólegar – feminine plural of rólegur
• Group of men only:
  
  • Þeir eru rólegir í dag.
    
    • þeir – they (masculine)
    • rólegir – masculine plural
• Mixed or neuter group (e.g. kids, animals, things):
  
  • Þau eru róleg í dag.
    
    • þau – they (neuter)
    • róleg – neuter plural

So you choose according to the natural gender of the group.

In Icelandic, you almost always use an adjective with vera (to be) to describe someone’s state:

• Hún er róleg í dag. – She is calm today.

An adverb (like rólega) is used to describe how someone does an action, not their state of being:

• Hún talar rólega. – She speaks calmly.
• Hún keyrir rólega. – She drives calmly.

So:

• Correct:
  
  • Hún er róleg í dag. – She is calm today.
• Not idiomatic:
  
  • ✗ Hún er rólega í dag.

Use the adjective (róleg), not the adverb (rólega), with er.

Er is the indicative form (stating a fact):

• Hún er róleg í dag. – She is calm today.

Sé is the subjunctive form, used in contexts like:

1. Reported / indirect speech or uncertainty:

   • Ég held að hún sé róleg í dag.
     
     • I think (that) she is calm today.

2. Conditionals, wishes, hypotheticals:

   • Ef hún sé róleg í dag, verður dagurinn auðveldur. (very formal/literary; spoken Icelandic more often keeps *er here)*

For a direct statement of fact like your original sentence, you use er, not sé.

Hún er róleg í dag. is completely neutral:

• Suitable for everyday conversation
• Also fine in more formal contexts

It uses basic vocabulary and standard grammar. You can say this naturally to friends, family, colleagues, or in written Icelandic.