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I am doing some Latin exercises and the sentences are given as the following (by Rosetta Stone):
Puer plus lactis habet quam vir.
Femina plures canes habet quam vir.
In the first case ("more milk") the object is in the genitive, but in the second case ("more dogs") the object is in the accusative. Why would these two parallel sentences differ in this way?
Asteroides's answer is spot-on, but since the comments express confusion with it, I'll try explaining it a different way.
Much like you wouldn't say *"the boy has more milks" in English, you wouldn't say *plura lacta in Latin. (You could perhaps use the plural if you wanted to talk about a café with many different types of milk, but it still sounds strange to me.)
The problem is, the adjective plurēs/plura doesn't have any singular forms, since semantically it's always referring to a large amount of something. So you can't say *plus lac, with an adjective, either.
So Latin uses a different construction in this case: the neuter noun plus with a genitive of quantity. You can think of it as "a larger amount": the boy has "a larger amount of milk".
For canis, though, there's no reason to say "a larger amount": there's a plural canēs "dogs", so you can just modify this with an adjective to get plurēs canēs, "more dogs".
This is the standard way to talk about "more" of a count noun, though you can also use plus with a genitive if you really want to: plus hostium "a larger amount of enemies".
Plus is used as an adverb, adjective and noun. The adjective is used only with plural nouns, so the difference between your sentences is because milk (lac, lactis) is singular and so cannot be used with an adjective form of plus but dogs (canes) is plural.
The adverb has the invariable form plus (Latin adverbs are invariable as a rule and quite often have the form of neuter singular nouns).
The adjective has the plural forms plures, plura, plurium, pluribus. It agrees in gender and case with a plural noun, which takes the usual case for its function in the sentence, to form an expression meaning "more [plural noun]". This is the usage found in plures canes.
The neuter noun plus has the singular forms plus, pluris, plure. It is "defective" (unattested) in the dative singular. The neuter noun plus is used with a noun in the "partitive genitive" to form an expression meaning "more [singular or plural noun]". This is the usage found in plus lactis.
Therefore, "plus lactis" is the only way to say "more milk" using plus. The whole expression "plus lactis" is accusative. The genitive portion is not inflected for the case of the whole expression because genitives can't be inflected any further for case in Latin, but the head of the phrase, plus, is inflected in case based on the function of the phrase, so it is accusative here.
"More dogs" in the accusative could and generally would be "plures canes" (I'm not sure if "plus canum" might be possible, but I don't think it was usual). Because "dogs" is plural, there is the option of using the construction where plus is an adjective. I think using plus as an adjective is the usual construction for plural nouns, but I have seen a few examples of plus with a plural genitive.
