Breakdown of Hver er heima? Enginn.
vera
to be
heima
at home
hver
who / what / which (one)
enginn
no one / nobody / no (m/f)
Questions & Answers about Hver er heima? Enginn.
Why is enginn used here instead of the neuter form ekkert?
When asking Hver? (Who?), you are asking about people. In Icelandic, the default gender for an unknown person is masculine. Because the answer stands in for the subject of the sentence (who is doing the being), it takes the nominative case. Therefore, we use the masculine nominative singular form, enginn.
Why do we use heima here instead of heim?
Heima indicates a stationary location (being at home), while heim shows movement (going home). Since the verb is er (is), there is no movement happening, so we must use the stationary location word heima.
Can enginn stand all alone as an answer like this?
Yes! Just like in English, you can drop the rest of the sentence. It is perfectly natural to just say Enginn instead of the full sentence Enginn er heima (No one is at home). The case of the pronoun stays nominative because it is still the subject of that hidden sentence.