Could the word "one" be "single" instead? I tried the latter, and got marked wrong.
Well, literally, it means "here one window", but sentences without a verb aren't allowed in English, so we have to infer one. "There is [something] here" is not actually contradictory in English, because "There is", in this instance, is not really referring to a place, but a state of being. "There is/there are" is a standard phrase in English, and not actually referring to WHERE anything is. You can say: "There is a man here", or: "There is a man on the moon". The actual location makes no difference to the "There is" construct, and certainly doesn't mean the subject can't be here. I'm no student of grammar or linguistics, so I can't tell you the proper technical term for this, although I'm sure there is one. But the English "there is" does not correspond to the Russian preposition: "там".