Learn Icelandic: Hún er að máta þessa kápu.

Þessa is the accusative (object) form of þessi for feminine nouns. Since the coat is the direct object receiving the action of being tried on, the word for "this" must be in the accusative case.

Why does the word for "coat" change to kápu?

The dictionary form of the word is kápa. Because it is the direct object of the verb máta (to try on), it changes to the accusative case. Feminine nouns that end in -a in their base form usually change to end in -u in the accusative.

Does the word for "this" always have to match the noun's case?

Yes! In Icelandic, a demonstrative pronoun like þessi must agree with its noun in gender, number, and case. Since the sentence needs the feminine accusative case, both words take their accusative forms together to make þessa kápu.

What does er að do in this sentence?

Icelandic uses the verb vera (to be) plus að and an infinitive verb to show an ongoing action, just like the English "-ing" form. Here, hún er (she is) plus að máta (to try on) means "she is trying on" right now.