Learn Hardcore Icelandic: Dagurinn er fullkominn.

dagurinn is the definite form of dagur (“day”), so it literally means “the day.” In Icelandic you don’t use a separate word for “the” – you add a suffix to the noun. For masculine nouns like dagur, the indefinite nominative is dagur, and the definite nominative is dagurinn.

What case, number and gender is dagurinn, and how can I tell?

• Case: nominative (it’s the subject of the sentence)
• Number: singular
• Gender: masculine
You can recognize the nominative masculine–singular definite by the -inn ending on dagurinn.

Why does the adjective fullkominn also end with -inn rather than something like -ur?

Adjectives in Icelandic agree with their noun in gender, number and definiteness. Here dagurinn is masculine, singular and definite, so fullkominn takes the masculine‐singular‐definite ending -inn. If the noun were indefinite masculine singular you’d see a different pattern for many adjectives (often -ur), but fullkominn is a special case (see next question).

Is fullkominn a past participle or an adjective? Can I break it down?

Historically it’s a compound full- (“fully”) + kominn (past participle of koma “to come”), but today it’s treated as a regular adjective meaning “perfect, complete.” You don’t conjugate it like a verb – you use it like any other descriptive adjective.

How would I say “The days are perfect” in Icelandic?

Dagarnir eru fullkomnir.
• Dagarnir = days (definite plural of dagur)
• eru = are (3rd‐person plural present of vera, “to be”)
• fullkomnir = masculine plural (definite) form of the adjective

How do I turn the statement into a yes/no question: “Is the day perfect?”

You invert the verb and subject:
Er dagurinn fullkominn?

How would I say “A day is perfect” (indefinite)?

Dagur er fullkominn.
Here dagur is indefinite (no -inn), and fullkominn stays the same because past-participle–based adjectives like fullkominn happen to look identical in the indefinite and definite masculine nominative singular.