When you use a preposition like в or на, you need to put the place in prepositional case.
Это мой стол - Nominative Яблоко на столе - Prepositional
You can read more grammar notes about this lesson on the Duolingo desktop website. They are pretty integral to helping understand the cases.
Prepositional Case. When you see the "HA" or when you see the "B" prepositions start looking for adjectives and nouns that blend correctly when you are translating them from Russian to English. The trick is hard to do at first, but then the magic happening suddenly shows up and you understand with practice why стол becomes столе.
That's right, technically, although I think the definite article is implied rather than the partitive, but anyway it's not common English usage. The objective here is to teach Russian not English; I think it's better to accept only common exemplars in the English translation because if not, the English-speaking learner may think that the equivalent Russian expression is somehow non-standard.
It's common enough where I'm from. Personally, I like the idea of duolingo being extremely lenient in evaluating english translations, since the objective is learning Russian, not English. It helps my comprehension/memorization to forgo impeccable grammar and structure sentences in English to better match the Russian syntax.
I see why you may think differently though!
Desk - письменный стол http://loftinterior.ru/img/catalogue/49728107002_600.jpg
Bread can not put on the desk. Mom will be disappointed.
How is it not a viable sentence?
"What bread are you referring to?" "The bread on the table"