Smjörið er mjúkt í dag.

to be

Breakdown of Smjörið er mjúkt í dag.

• í dag: today
• smjörið: the butter
• mjúkt: soft

Questions & Answers about Smjörið er mjúkt í dag.

Why does the word Smjörið end in -ið?

Icelandic uses a suffixed definite article. For neuter nouns, the nominative/accusative singular definite ending is -ið. So:

• smjör = butter
• smjörið = the butter (a specific, context-known butter, e.g., the butter in the fridge)

Why does the adjective appear as mjúkt and not something else?

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 (to be). is grammatically singular, so you use . is for plural subjects.