Learn Hardcore Icelandic: Hún klæðist fallegri flík í dag. - She is wearing a beautiful garment today.

klæðist is the third-person singular present of að klæðast, a middle-voice verb meaning “to wear” or “be dressed in.” Unlike English “to wear” (which takes a direct object in the accusative), klæðast takes its object (the clothing) in the dative case.

Why is fallegri flík in the dative case?

Because að klæðast governs the dative. All clothing items that someone “wears” with klæðast must be in dative. Hence flík (feminine noun) becomes fallegri flík (dative singular feminine).

Why is the adjective fallegur inflected as fallegri here?

Adjectives in Icelandic agree in gender, number and case with the noun they modify. flík is feminine and in the dative singular. In strong declension (no definite article), the dative singular feminine ending for fallegur is -ri, giving fallegri.

How would you say “the beautiful garment” instead of “a beautiful garment”?

You add the definite suffix -in on the noun. The feminine dative singular of flík with a definite article is flíkin, so you get fallegri flíkin.
Example: Hún klæðist fallegri flíkin í dag.

Could you use klæða sig í instead of klæðast? If so, what changes?

Yes. You can say:
Hún klæðir sig í fallega flík í dag.
Here fallega flík is accusative singular feminine because að klæða is a transitive verb taking the accusative object, and you use the reflexive pronoun sig (“herself”). Many speakers, however, prefer the more concise klæðast + dative.

Why is í dag written separately and not ídag?

í dag (“today”) is a fixed adverbial expression. Even though í generally takes the dative for location, in this time-expression it remains dag in the accusative (not degi). That’s just the standard spelling.

Can you move í dag to the beginning of the sentence, and how does word order change?

Yes. Icelandic word order is relatively flexible. If you put í dag first, the finite verb still occupies the second position:
Í dag klæðist hún fallegri flík.
This fronting emphasizes “today.”

Why isn’t there an English-style article (“a” or “the”) before flík?

Icelandic has no separate indefinite article; indefiniteness is unmarked. “A beautiful garment” is simply fallegri flík. To make it definite you add a suffix (e.g. flíkin = “the garment”).