Questions & Answers about Ég er vanur að borða mikið prótein.
Why does the sentence use vanur instead of another form like vön?
Because vanur is an adjective, it must agree with the subject in gender and number. Vanur is the masculine singular form. If the speaker were female, she would say Ég er vön að borða..., and a mixed group would say Við erum vön að borða....
The English translation says 'eating'. Why is the infinitive að borða used here?
In Icelandic, the habit of doing something is expressed with vera vanur + að + the infinitive verb. Icelandic doesn't have a direct equivalent to the English '-ing' gerund form, so it always relies on the infinitive like að borða (to eat) to express this kind of habitual action.
Why is it mikið prótein rather than taking a dative form like miklu próteini?
The case of mikið prótein is determined by the verb borða (to eat), which takes the accusative case. Even though some adjectives force a dative case, here the noun phrase is simply the direct object of borða. Because prótein is a neuter noun, its accusative form looks identical to its nominative form, and mikið matches it.
How would I change this sentence to say 'I was used to eating a lot of protein'?