Learn Polish: Na górze jest okno. - There is a window upstairs.

While góra does mean "mountain", it also means "the top". So, na górze literally means "on the top". When talking about a house or building, Polish uses this exact phrase to mean "upstairs".

Why does the spelling change to górze instead of góra?

It changes because it follows the preposition na (on/at) to describe a location. This triggers the Locative case. For feminine nouns ending in -ra like góra, the ending transforms into -rze.

The translation says "There is", but where is the Polish word for "there"?

English uses a dummy word in the phrase "there is", but Polish does not. To say "there is a window", you simply use the verb jest (is). The sentence literally translates to "On the top is a window."

Is it okay to say Okno jest na górze instead?

Yes, but it changes the meaning slightly. Polish word order puts the most important new information at the end. Na górze jest okno answers "What is upstairs?" (There is a window upstairs). Okno jest na górze answers "Where is the window?" (The window is upstairs).