That's the partitive case ending. We use it with objects and predicatives when we do not know whether they are complete or not (Because it's called the PARTitive case. ahem). Uncountable nouns like "milk" fall into this group always. If the word is an object, you can usually add "some" or "any" in front of it depending on whether it's in a statement or a question.
- Minulla on maitoa. I have (some) milk.
- Onko sinulla maitoa. Do you have (any) milk?
- Onko tuo maitoa? Is that milk?
Standard Finnish does not really have a structure to express it that shortly. In spoken language, however, you can use the pronoun se a bit like a definite article (although it's not really one). You would still use the partitive in most cases, but the nominative is also possible, if you are talking about a one single, complete carton/bottle.
- Onko tämä se maito? Is this the (same) milk (you bought earlier and it's all still there)?
- Onko tämä sitä maitoa? Is this the (same) milk (you bought earlier and it's only some of it or I do not know whether it's all still there)?
sitä is the partitive form of se. If this seems complicated, do not worry about it too much yet. Being able to make this distinction in this type of structure is so advanced Finnish that we do not even include it in the course yet! :)
Personally, I would just use the word sama(a) here anyway:
- Onko tämä samaa maitoa? Is this (some of) the same milk?
- Onko tämä sama maito? Is this the same (carton/bottle/glass of) milk?
Being able to make this distinction in this type of struncture is so advanced Finnish that we do not even include it in the course yet! :)
Thank you for that! :D
So basically, we always need to know the specific context in order to decide whether the nominative or the partitive form is correct. (Apart from the instances where the partitive is always required such as “kolmea mustaa koiraa”.)
"Minulla on maitoa. I have (some) milk. Onko sinulla maitoa. Do you have (any) milk? Onko tuo maitoa? Is that milk?" I can understand this, but some examples earlier in this lesson (Food, I think it is level 2) have what seem to be similar sentences but it is simply maito or juusto. This is confusing. I cannot paste them here because I cannot go back to see them.