Learn Icelandic: Hún var vön að minnka kolvetnin. - She was used to reducing carbohydrates.

In the habitual construction vera vanur að (to be used to), vanur functions as an adjective. This means it must agree in gender, number, and case with the subject. Since the subject is Hún (a feminine, singular pronoun in the nominative case), the adjective takes the feminine nominative singular form vön.

Is there a difference in meaning between using var vön að minnka and just using the simple past minnkaði?

Yes. While hún minnkaði kolvetnin just means 'she reduced the carbs' (which could be a single event), using the var vön að construction explicitly highlights that this was an ongoing habit or routine she maintained in the past.

Why is kolvetnin in the definite form ('the carbohydrates') when the English translation says 'reducing carbohydrates'?

Icelandic often uses the definite form when talking about general categories, substances, or diet components in a broad sense. While English drops the 'the' for general plural nouns, Icelandic keeps the definite suffix to refer to the concept of carbs as a whole.

What case does the verb minnka trigger for kolvetnin, and how can we tell?

The verb minnka governs the accusative case for its direct object. The word kolvetni is a neuter noun, and neuter nouns always have the exact same form in the nominative and accusative cases. The -n at the end is the definite article for the neuter plural, showing it is indeed accusative plural here.