Translation:I order one liter of soup and two kilograms of tomatoes.
"Da" is a preposition. Words with a preposition (generally) don't get an -n.
Although the -n can be used to mark movement or direction:
en la domo = inside the house
en la domon = into the house)
But that's a lesson for later in the course. Until it comes up just stick to the rule of thumb that words with a preposition don't get an -n.
Why does "tomatoj" have the plural ending, while "supo" does not? I believe it is because soup is continuous, and tomatoes are not.
However I think my question stems from the fact that in talking about cooking, it sounds more natural to me to say: "The recipe calls for two kilograms of tomato" than it would be to end that sentence with "two kilograms of tomatoes". Is my first sentence, then, colloquially used but grammatically incorrect English? Does the answer change when the tomatoes called for are whole, as in my given example, vs chopped ? (I presume if they were pureed then they would be considered continuous, like soup, so if they start out as countable, that is-noncontinuous, and then as you chop or smash them at some point they are considered continuous...how is that point determined in Esperanto, heck, even an answer on how the rules say it should be determined in English would be helpful).
(I am a native English speaker.)
I am also a native speaker, and I think it's not very clear cut. Labels at the grocery store definitely say "Diced Tomatoes." It doesn't really sound wrong to say "a bowl of diced tomato." However, I would always say "a bowl of diced tomatoes," since it (most likely) took more than one tomato.
Not necessarily, because "unu" means one, while having no article before a noun, such as "litro" means "a liter". I guess it would mean the same thing, "a liter" or "one liter", but if you're told to translate "one liter of water" you are told that it's "one" and not "a" and therefore need to put "unu litro da akvon". If you're given "litro da akvon" then you know to put "a liter of water" rather than "one liter of water" because without the article in front of "litro" it is automatically "a" or "an"
The grammatical case that expresses part of something, is called partitive. Partitive is expressed in many Indo-European languages by a prepositional construct, in English by the word of , one litre of soup. E-o basing on Indo-European languages uses da, unu litro da supo.
Take a look at my response above, where I talk about the partitive object.
Generally speaking "da" is used to add a measure to a thing. The thing is usually non-countable or non-discrete. PIV (Plena Ilustrita Vortaro) gives two uses:
1) prepozicio esprimanta kunon de fizika aŭ metafora ujo kaj ĝia enhavo
glaso da akvo: a glass of water (with subunderstanding that the glass is completely filled with water)
60 paĝoj da teksto: 60 pages of text (not counting empty pages or pages exclusively with illustrations)
tuto da kondiĉoj kaj ĉirkaŭaĵoj: (taking into account) all conditions and circumstances
2) prepozicio uzata por rilatigi kun iliaj komplementoj vortojn esprimantajn precizan aŭ neprecizan kvanton, nombron, mezuron, pezon.
unu kilogramo da viando: one kilogram of meat
pli da akvo: more water
"De" is a multipurpose beast with over ten different meanings (of, from, by...). See PIV.
PS. There are also cases where both "da" and "de" can be used, but they have different meanings.