Questions & Answers about Mig vantar þennan part og eitt afrit af honum.
Why is the sentence using Mig vantar instead of Ég vanta?
Vanta (to need or lack) is an impersonal verb. The person who needs something is treated grammatically as the object, so you use the accusative case mig instead of the nominative ég. The thing that is needed is also in the accusative case.
How does vantar affect the words þennan part and eitt afrit?
Because vantar forces the needed items into the accusative case, both þennan part (this part, masculine accusative) and eitt afrit (one copy, neuter accusative) take the accusative form.
Is eitt used here as part of the einn... annar (one... the other) structure?
Not in this specific sentence. Here, eitt simply means the number one. However, it declines exactly the same way when used in the einn... annar pattern, taking the gender and case of the noun it relates to (neuter accusative here to match afrit).
Why does the sentence end with af honum if it refers to an inanimate part?
In Icelandic, pronouns must match the grammatical gender of the noun they replace. Because partur (part) is a masculine noun, you use the masculine pronoun hann. The preposition af requires the dative case, changing hann to honum.