Breakdown of Sölumaðurinn sýnir ekki bara vasahnífinn, heldur líka tólið.
ekki
not
bara
just / only
sýna
to show
líka
also
Questions & Answers about Sölumaðurinn sýnir ekki bara vasahnífinn, heldur líka tólið.
How does the ekki bara... heldur líka structure affect the cases of vasahnífinn and tólið?
This correlative conjunction links the two nouns, meaning they share the exact same grammatical role in the sentence. Since sýnir (shows) requires its direct object to be in the accusative case, both vasahnífinn and tólið must be in the accusative.
Is the word líka strictly required in this sentence?
It is highly recommended but occasionally dropped. Just like in English, you could technically say "not only the knife, but the tool" (ekki bara... heldur), but adding líka (also) makes the sentence feel much more balanced and natural.
If I wanted to add "shows me" to this sentence, what case would "me" take?
It would take the dative case. The verb sýna takes the dative for the person being shown something, and the accusative for the item being shown. You would say Sölumaðurinn sýnir mér... where mér is dative.
Both vasahnífinn and tólið are definite and in the accusative case. Why do their endings look so different?
This comes down to noun gender. Vasahnífur is a masculine noun, so its definite accusative suffix is -inn. Tól is a neuter noun, so its definite accusative suffix is -ið.