"There is a fly on the stone."
Translation:На камне муха.
"Есть муха на камне" sounds like a reply in an argument:
- На камне муха. (There's a fly on the stone.)
- Нет там мухи. (No, there isn't any fly there.)
- Есть! (Yes, there is!)
- Да нет мухи на камне! (Come on, there's no fly on the stone!)
- Есть муха на камне!! (Yes there is a fly on the stone!!)
The most neutral way to say "There is X [somewhere]" in Russian is to say "[somewhere] X".
In many other "there is/are" sentences we've looked at, есть has been required in declaratory sentences, so I'm kind of surprised есть is not required here.
(I wish there was an easy way to access my comment history, as used to be the case with Duo with our now long-gone personal pages, or the search engine was a lot better, so I could locate the sentences I'm talking about.)
Example from a Duo exercise I found using the search engine:
"There is an apple on the table."
Есть яблоко на этом столе.
Besides the infuriating use of "the" to translate этом (I've been marked wrong numerous times for doing that), the example makes it likely that I'd write:
есть муха на камне
Your comment makes it seem that the actual "correct" answer (without any context, there seems to be more than one) to the "apple/table" sentence is:
на этом столе яблоко
на этом столе — яблоко.
I'd recommend using есть for "there is" in general situations. When you want to say "There is [something] [somewhere]," the construction Duo is teaching here is the best, most neutral, and natural...[Locative/ prepositional case of place] [the thing which is, in nominative]. For instance: На столе тарелка. This is best rendered in English as "There is a plate on the table." An example of when you'd want есть would be when you want to stress the existence of something (Я верю, что добро есть...I believe that there is good.) Or when you are just generally saying "there is" with no location: "Ходите в свете, пока есть свет" (Walk In the Light While There Is Light...title of a Tolstoy novella). Even the second example is stressing the existence of the light. Hope that helps :)
The "e" is a fill vowel, which disappears in declension except for the accusative singular (same as the nominative singular). In the same way, to break up consonant clusters at the end of words, many neuter and feminine nouns add such fill vowels in the genitive plural.
It comes from an English speaker's need for the sentence to sound sensible.
Otherwise, it is not there. Duo wants to know if you understand the sentence so it expects something closer to English than just a simple word by word translation. If you were translating the Russian into a language that didn't require the verb to be in an ordinary sentence, you wouldn't have to include it.
Both of these are possible in English as well, with slightly differing emphasis: "On the stone is a fly" stresses what's happening on the stone. "The fly is on the stone" suggests that we're interested in the fly and it's current whereabouts. What Russian lacks is the distinction between the definite and indefinite articles, since it has neither. In English "A fly is on the stone" gives the general state of things, where neither the fly nor the stone is of particular importance; whereas "The fly is on the stone" means we're already aware there's a fly and we're reporting on it's current situation.
The difference is nuanced. This emphasizes the fly's location, as though we already know of the fly and are inquiring where to find it. "The fly is located (finds itself) on the stone."
The exercise question presents the existence of a fly as new information, or perhaps answers the question, "What's on the stone?"