Translation:Vera, boil the mushrooms and the potatoes.
Yes, I hear «карто́льку» too. :) It's not how it should be pronounced.
This isn't a true Russian sentence. We say, "Приготовь картошку с грибами" or, if we want to be more specific, we say, "Отвари и пожарь грибы, свари картошку и мы поедим картошку с грибами". Nobody boils mushrooms and potatoes together, indeed. Moreover, mushrooms are always cooked on a frying pan, unless they are pickled. If you ask me about the difference between "свари" and "отвари", I'll have to admit it is really subtle, but I think "свари" is used to emphasize the idea of making a dish ready for consumption, whereas "отвари" refers to boiling as a part of a longer cooking process and is therefore used for ingredients rather than the whole thing.
I think singular "Potato" should be accepted.
I know that you would cook the potatoes in English, rather than one potato, but in some other exercise if I would actually make that "correction", to make the sentence in English to actually sound natural, you would mark it as incorrect.
You guys just aren't consistent with it.
Sometimes you would go with a literal translation, which you would never say in English, sometimes you insist on completely "poetic" and idiomatic phrasing like you grew up half of the childhood in East Side London and another half near Shelkovskaya Station in Moscow
While I don't use it in plural, some speakers here on Duolingo have said that they use «картошка» to mean a single potato (i.e. as a synonym of «картофелина»), and this meaning is found in dictionaries: http://gramota.ru/slovari/dic/?word=картошка&all=x
So it technically can have a plural form (although I haven't heard it used myself).
Because there are ao many potatoes in russia they became uncountable nouns.
But seriously, i think in many languages vegetablea that you cut up into small pieces become uncountable and are refered to in the singular.
Think: When cooking, how could you count out exactly 2 potatoes from your pile of bits? So we use a poetic singular.
Singular nominative/accusative: гриб
Plural nominative/accusative: грибы́
Only nouns ending in -а/-я in nominative have a separate accusative form (e.g. nominative вода́ 'water', genitive воды́, accusative во́ду). Usually these nouns are feminine, but masculine nouns also use this set of endings (nom. па́па 'Dad', gen. па́пы, acc. па́пу).
For all the other nouns (and for plural nouns), accusative is either same as nominative (e.g. nom. сто́л 'table', gen. стола́, acc. сто́л), or same as genitive (e.g. nom. сло́н 'elephant', gen. слона́, acc. слона́). If the noun is animate (describes a living being), then accusative is same as genitive. If it's inanimate, then accusative is same as nominative.
Boil mushrooms? Not even the Brits do that to food anymore. Of course, after Brexit, they may revert to boiling everything.
"A French chef can make shoe-leather taste like beef, and an English chef can make beef taste like shoe-leather."
but it all starts with boiling mushrooms.