Translation:The subway is on the left, and the park on the right.
Since it's capitalized, it must be referring to the sandwich chain (but probably that's just another error).
PS: I didn't even know that Subway existed in Britain! Do they still put old-fashioned maps of the New York subway system on the walls there, or do they use a London tube map instead?
Perhaps worth noting that "l for left, and r for right" is common in many languages, such as "links" and "rechts" in German. "Слева" and "Справа" are each two words, with the roots being "лево" and "право" with a prepositional "С" bolted onto the front. Of course, "право" ("right") is related to "правда"("truth")....
There is no verb in your sentence. You could say, "The metro is on the left, and a park is on the right.
Also, "the metro" is almost always referred to with the definite article "the," not indefinite article "a." An example of the latter is, "Paris has a metro; the metro is over there on the right."
Well, the problem wasn't with the word "underground"(it is already accepted) but with the translation of "а", that could be "and" or "but"... However, it seems this second option is, perhaps, ungrammatical, or not correct in this context, or... Could anybody explain it, please?
"a" means "and" here. The English could be either "The metro is on the left, and the park is on the right" or "The metro is on the left, but the park is on the right". Which one depends on context.
"a" is used where there is contrast but not conflict; if conflict between two clauses exists, then "но" is used.
I appreciate the work the makers of this course are doing, but honestly! - The rigidity of what they determine to be right vs. wrong answers makes learning frustrating. I wrote "The Metro is to the left, and the park to the right." This was marked wrong, as so many similar answers have been.
The "it" in your sentence is unnecessary and is not natural English. "On the left is the subway" is correct English, omitting "it".
I can understand your concern here, because there is an understood "it" or "there" which is not stated. If you wanted to include something like that, then you would say, "On the left there is the subway" - somewhat awkward English, but closer to what I think you're trying to say.
Please observe, however, that "it" = это does not appear in the Russian sentence. Most of the sentence I've seen where the English starts with "This is [something]" or "It is [something]", the Russian is usually "Это [something], e.g., "это машина" = "it is an automobile".
Since это is not in the Russian, then "it" shouldn't be in the English.