"Corinna builds four cities."
Translation:Corinna quattuor urbes condit.
The problem is in English. "Build a city" is allowed in English to stand for what is actually done, which is "found a city." In Latin, you don't construis a city; you construis buildings, or furniture, or physical things. Cities, countries and corporations, which are at the end of the day abstract legal entities, conduntur, that is to say, "are founded."
"construere" is more like "to construct" (a building or other structure) "condere" is more like "to found or establish" (a city or other place)
Both can be reasonably translated in English by "to build", but they have noticeably distinct meanings.
The traditional year count for dates in ancient Rome was such-and-such years "ab urbe condita" -- "from the founding of the city [of Rome]".
I'm gonnaaaaa assume delcintions was meant to be declensions, and say that it should be urbes because it's 3rd declension. Urbem is the singular accusative of the noun. Urbes is both the nominative plural and accusative plural of the noun. I'm gonna give you a little tip for more in-depth Latin learning: some nouns may have the same form as another case, so you'll have to use all words for more context about it. To figure out whether the noun urbes is accusative or plural in a random sentence, think about the verb(s) and other noun(s). Take multi iuvenes urbes delent, for example. You can tell that, by the order of the sentence, multi applies to the noun iuvenes. This must be the subject of the sentence because multi is nominative and there is no linking verb to make it a predicate nominative. Therefore, urbes will be the direct object of the sentence. You can also look at the verb as it may have to do with the number of the subject. Keep Latin-ing!!
Nominative (subject): urbs/urbēs
Accusative (direct object): urbem/urbēs
Dative (indirect object): urbī/urbibus
Ablative (instrumental + other meanings): urbe/urbibus
Genitive (possession): urbis/urbium
So, urbe cannot be the subject, unless it is a sentence in so-called "ablative absolute," which eventually the course should cover. However, if you meant nominative urbs, then it is the same as we have in this sentence and identical to the accusative plural (both urbēs).