Questions & Answers about Hún fyllti glasið af köldu vatni.
Why do we use af here instead of með to say 'with'?
It is a set phrase. In English, you fill something with something, but in Icelandic, the verb fylla requires the preposition af. The grammatical pattern to memorize is fylla eitthvað af einhverju (to fill something of/with something).
Why is glasið in the accusative case while köldu vatni is in the dative?
The verb fylla requires its direct object (the container being filled) to be in the accusative case, which gives us glasið. The contents (köldu vatni) are governed by the preposition af, which always forces the words following it into the dative case.
Why is the adjective köldu rather than kalt?
Adjectives must agree with their nouns in gender, number, and case. Vatn is a neuter noun, and here it is in the dative singular. The dative neuter singular form of kaldur is köldu. We use the strong declension because there is no definite word (like 'the' or 'this') directly in front of 'cold water'.
How would you rephrase this to say 'The glass filled with cold water' (without mentioning who filled it)?
You would drop hún and use the middle voice form of the verb, which adds -st to the end. The sentence becomes Glasið fylltist af köldu vatni. The middle voice is commonly used in Icelandic for actions that seem to happen on their own.