Questions & Answers about Við kaupum ost og fisk.
Why are the words for cheese and fish written as ost and fisk instead of ostur and fiskur?
In this sentence, they are the direct objects (the things being bought). The verb kaupa forces its objects into the accusative case. For many masculine nouns like ostur and fiskur, you form the accusative simply by dropping the -ur ending.
Why does the verb kaupa become kaupum here?
The ending of the verb changes to match the subject. Whenever the subject is við (we), the verb almost always takes the -um ending.
Do both nouns connected by og have to be in the accusative case?
Yes! Because you are buying both items, they are both direct objects of the verb. The grammatical case applies to everything on the receiving end of the action, so both words drop their -ur endings.
Is there a missing word for 'some' before the cheese and fish?
Icelandic does not use words like 'a', 'an', or 'some' before nouns the way English does. A bare noun like ost translates naturally to 'cheese' or 'some cheese' depending on what makes the most sense in English.