"The young man wants to descend from the altar."
Translation:De ara iuvenis descendere vult.
"The stream is flowing beneath the bridge": you can analyze this as sub + ablative indicating 'location.'
You can see sub + accusative in some situations, but it's by no means as common as in + accusative.
Maybe for "Let's go hide it under the bridge," you'd see sub + accusative. (The act of moving something under something else is being highlighted here.)
Some prepositions take exactly one case, whereas others can take more than one.
Examples of prepositions which only take one case:
- "a/ab", "de", and "cum" always take the ablative case.
- "ad", "per", "ante", and "post" always take the accusative case.
Examples of prepositions which can take more than one case:
- "in" and "sub" take the accusative case if movement into/towards something is involved; otherwise they take the ablative case.
(If I'm wrong here, someone please correct me.)
In this translation exercise, since "de" is used, we use the ablative case, as "de" always takes the ablative case in Latin.
But there are phrases like ex equō dēlābī (to fall off a horse, Livy), in which the person is surely no more 'inside' the horse than the putative young man is 'inside' the Duolingo altar.
There are some phrases in the OLD that are said to illustrate the meaning "from the surface of, from off" and "down from": including, in Ovid's Metamorphoses (1.261) "ex omnī nimbōs dēmittere caelō" (to send down clouds from the whole sky); note the combination of prep. ex even with a verb compounded with dē- . And Cicero has a sentence about someone hurling himself ex the highest wall ( sē ex altissimō praecipitāsse mūrō). Looking under ab, there do seem to be instances of ab + places with the meaning of "away from."
Yes, it's important to realize that not every masculine noun in Latin will end in -us in the nominative singular (just as not every feminine noun will end in -a, either!).
"Young man" is a 3rd declension noun, with nominative ending -is in the singular, as Copernicus explains.
You would see it listed as: iuvenis, iuvenis , m., young man, from which you can tell that it belongs to the 3rd declension (since its genitive sing., the 2nd form listed, ends in -is).
Iuvenis laetus est . The young man is happy. Since it's a masculine nominative singular noun, notice that it is modified by an adjective that ends in -us.