Learn Hardcore Icelandic: Smjörið er mjúkt í dag.

Adjectives agree with the noun in gender, number, and case. Smjör is neuter singular, so the adjective must be neuter singular nominative. The base adjective is:

masculine: mjúkur
feminine: mjúk
neuter: mjúkt Here, we need the neuter form: mjúkt.

But the noun is definite—shouldn’t the adjective be weak (like mjúka)?

Not in this position. After the linking verb vera (to be), adjectives used predicatively take the strong endings, even if the noun is definite: Smjörið er mjúkt.
You get the weak form when the adjective is attributive before a definite noun: mjúka smjörið = the soft butter.

Why is it er and not eru?

Er is the 3rd person singular present of vera (to be). Smjörið is grammatically singular, so you use er. Eru is for plural subjects.

What cases are used in the sentence?

Smjörið: nominative singular (subject)
mjúkt: nominative singular (agrees with the subject)
dag in í dag: accusative singular (time expression)

Does the preposition í take the accusative or the dative?

Both, depending on meaning:

Location (in/at): dative. Example: í ísskápnum (in the fridge).
Motion into or time spans/points: accusative. Examples: í ísskápinn (into the fridge), í dag (today).
So in í dag, dag is accusative.

Can I move í dag to the front of the sentence?

Yes. Icelandic is a verb-second (V2) language in main clauses. If you front an adverbial like í dag, the verb stays in second position:
Í dag er smjörið mjúkt.

Can I leave out the definite article and say Smjör er mjúkt í dag?

Grammatically yes, but it changes the meaning. Smjör er mjúkt í dag sounds like a generic, “butter (in general) is soft today,” which is unusual. In real-life contexts you typically mean a specific butter, so Smjörið is the natural choice.

How do I pronounce Smjörið?

Approximate it as “SMYUR-ith.” Details:

smj ≈ “smy” (with a short y-glide)
ö ≈ a rounded vowel like the vowel in British English “burn” (closer to [œ])
r is tapped/trilled
final ð is like the “th” in “this”
IPA (approx.): [ˈsmjœːrɪð], with stress on the first syllable and a long ö.

How do I pronounce mjúkt?

Roughly “MYOOKT.”

mj ≈ “my”
ú is long [uː] like “oo” in “goose”
kt is pronounced as a voiceless cluster (often [xt]-like for many speakers)

What’s the gender and declension of smjör?

It’s neuter and typically uncountable. Common singular forms:

Indefinite: nominative/accusative smjör, dative smjöri, genitive smjörs
Definite: nominative/accusative smjörið, dative smjörinu, genitive smjörsins

Why is there no separate word for “the” before Smjörið?

Because Icelandic normally attaches the definite article to the noun as a suffix (here -ið). There is a separate article (hið for neuter), but it’s used in specific, often more formal or stylistic contexts (e.g., hið mjúka smjör).

Why is it í dag and not á dag?

The idiomatic expression for “today” is í dag. Prepositions differ by set phrases:

í dag (today)
á mánudaginn (on Monday; accusative for a specific Monday) These are fixed usages you learn with each expression.

Is there a difference between í dag and núna?

Yes:

í dag = today (the whole day as a time frame)
núna = now (this moment)
You can say Smjörið er mjúkt núna (the butter is soft now), which emphasizes the present moment rather than the day as a whole.

How would I ask, “Is the butter soft today?”

Invert the verb and subject: Er smjörið mjúkt í dag? (Yes–no questions put the verb first.)

How would I say, “The butter was soft yesterday”?

Use the past of vera and the time word for “yesterday”: Smjörið var mjúkt í gær.

I saw mýkt somewhere—can I use that here?

No. Mýkt is a noun meaning “softness.” In this sentence you need the adjective form agreeing with the noun: mjúkt (neuter singular).