Questions & Answers about Hún borðaði minna þegar hún var veik.
Is minna an adverb or an adjective here?
It is an adverb. minna is the comparative form of the adverb lítið (little). Because it describes how much she ate (the verb borðaði), it acts as an adverb of comparison meaning "less".
Could I use minni instead of minna in this sentence?
No, because minni is an adjective used to describe a noun (for example, a "smaller" house). Since we are describing the action of eating, we need the adverb form minna.
Why does the sentence use þegar for "when" instead of hvenær?
We use þegar to connect two parts of a sentence, meaning "at the time that". The word hvenær is strictly a question word used to ask "when?" (as in, "When did she get sick?").
Why is the word for sick veik instead of veikur?
Adjectives in Icelandic change to match the noun they describe. Because veik describes hún (she), it takes the feminine singular form. The default dictionary form is masculine (veikur), but the feminine form drops the -ur ending.