"Red wine costs thirty coins."
Translation:Vinum rubrum triginta nummis constat.
I had the same thought at first. Duolingo doesn't do a good job of explaining that for a few verbs, what looks like the direct object is dative case instead of accusative. "Consto" (cost) is one of them; other are "studeo" (study), near the beginning, and "appropinquo" (approach), which shows up a bit later on.
There's a list at the bottom of
A little more context can be found at
Even more context at
Edit (21 April 2020)
On reflection, I'm pretty sure that "constant" in this sentence is taking the ablative, not the dative.
(The ablative plural and dative plural of "nummus" look the same: "nummīs".)
Cōnstō, -āre, when it means "to be consistent with" can take the dative.
Cōnstō, -āre, when it means "cost" can take the ablative of price.
Cōnstō, -āre, when it means "cost" can take also take the genitive of indefinite value, as in Cicero's letter to Atticus:
"prope dimidio minoris constabit isto loco." -- "it will cost hardly more than half as much in that locality." ("minoris" is in the genitive)
Anticipating the question, here's a list of verbs that take the genitive:
May as well mention that there are a number of verbs that regularly take the ablative of separation:
Red wine is a nominative in the sentence, it is just a neuter second declension noun. Being neuter changes the endings as such.<pre>
Nominative -um , -a
Dative -i , -orum
Genitive -o , -is
Accusative -um , -a
Ablative -o , -is
The endings for the accusative and nominative are the same, so you have to decide what fits best in the sentence. Nominative nouns can either be the subject or predicate nominative, and Accusative nouns can either be the direct object (or recipient of an action) in a sentence or an object of the preposition (it is that form with certain prepositions). In this case, 'vinum rubrum' or red wine is the subject of the sentence, so it is nominative.
It is not vinum ruber because ruber is an adjective (meaning red) and it takes the same gender, case, and function as the noun it influences. To show this, people use the line 'ruber, -a, -um' to show the way the adjective is changed in ending based on the gender of the noun it modifies. The adjective would be seen as ruber for masculine nominative form, rubra for feminine nominative form, and rubrum for neuter nominative form. Ruber would modify a nominative masculine subject. Vinum is a neuter noun though, so the adjective must be changed to agree with the noun it modifies. 'Rubrum' is the neuter nominative adjective, and it agrees with the subject. It is the correct form.