Learn Icelandic: Grænar baunir eru góðar með fiski.

They are both describing the noun baunir (beans), which is a feminine plural noun in the nominative case. In Icelandic, all adjectives modifying a noun, including predicate adjectives like góðar that come after the verb, must agree with the noun in gender, number, and case. The feminine nominative plural ending for strong adjectives is -ar.

Why is the word for fish fiski instead of the dictionary form fiskur?

The preposition með forces the following noun into a specific case. When með means together with or accompanying, as in serving one food with another, it requires the dative case. Fiskur is a strong masculine noun, and its dative singular form drops the -ur ending and adds an -i, becoming fiski.

I learned that með can also take the accusative case. Why is it dative here?

That is a great catch! The preposition með takes the accusative case when it implies carrying, holding, or bringing something. However, when it means being accompanied by something, using a tool, or eating foods together, it takes the dative case. Since the beans are accompanying the fish on a plate, dative is used.

Why is there no definite article on Grænar baunir or fiski?