Learn Icelandic: Þetta er ekki bara vasahnífur, heldur líka vopn. - This is not only a pocket knife, but also a weapon.

They are both subject complements following the verb er (to be), which always takes the nominative case. The correlative conjunction ekki bara... heldur líka links these two nouns, meaning they must match in case. If the main verb were an action verb requiring the accusative, both of these nouns would be in the accusative case instead.

Is the word líka mandatory in this structure?

No, you can drop it and just say ...ekki bara vasahnífur, heldur vopn. Adding líka emphasizes the additive nature of the sentence (not only X, but also Y). Without it, heldur alone usually translates closer to 'but rather' or 'but instead', though the presence of ekki bara early in the sentence already makes the additive meaning clear.

Why does the sentence use the neuter pronoun þetta when vasahnífur is a masculine noun?

When you point to an object or introduce a concept using the 'this is...' structure, Icelandic requires the neuter demonstrative pronoun þetta, completely regardless of the grammatical gender of the noun that comes after the verb. You only use the masculine þessi if you are modifying the noun directly, such as in the phrase þessi vasahnífur er... (this pocket knife is...).

How is the compound word vasahnífur formed?