Marcus urbēs nōn condit. = Marcus does not found cities. -- "urbem" is accusative singular, "urbēs" is accusative plural -- accusative case because direct object of verb -- long vowel markers, BTW, are of potential importance for understanding poetic meters, and often for understanding which syllables should be accentuated in longer words --
Yes, "Marcus does not build the city" is correct... but not because of "urbem".
In Latin, there are no articles, so you are left to chose to use wether "the" or "a" (depending from the context, your taste...)
Latin is a language with declensions: the ending of nouns (and adjectives) changes according to their grammatical function in the sentence. There are many functions attached to each case, but in general:
- nominative: subject, predicative => "Rome is a city" / "Roma URBS est"
- vocative: direct adress => "City, you are beautiful" / "URBS, pulchra es"
- accusative: direct object => "He builds a/the city" / "URBEM condit"
- genitive: attributive => "Romulus is the creator of the city" / "Romulus creator URBIS est"
- dative: indirect object => "I write a letter to the city" / "URBI epistulam scribo"
- ablative: origin of a movement and... many things => "I come from the city" / "Ab URBE venio"