"Surely Marcus does not live alone?"
Translation:Num Marcus solus habitat?
In the English sentence I see "alone" as an adverb, and if it served the same role in the Latin sentence I would expect it not to decline. I can't find anything saying "solus" has an adverb form, so I'm curious if the sentence is less like the English (Marcus (lives alone)) and more like ((lone Marcus) lives), or whether the adjective actually does qualify the verb, but continues to decline as an adjective.
Marce is the vocative case while Marcus is the nominative. We use the vocative when the sentence is being addressed to Marcus, when he is the 'you' being talked to.
Marce, quid agis? -> "Marcus, how are you?"
Marce, ubi es? -> "Marcus, where are you?"
Marce et Livia, ubi estis? -> "Marcus and Livia, where are you?"
We use the nominative when Marcus is the subject of the sentence and we are not addressing him directly, when he is the 'he' or apart of the 'they' of the sentence.
Quid agit Marcus? -> "How is Marcus doing?"
Ubi est Marcus? -> "Where is Marcus?"
Ubi sunt Marcus et Livia? -> "Where are Marcus and Livia?"
The vocative is normally the same as the nominative but 2nd declension nouns that have a nominative that end in -us have the ending -e for the vocative. There are some other vocative endings but not currently used in the course.