"It is a good party."
Translation:To jest dobra impreza.
It's definitely a correct translation, even if the meaning's broader. I think the first connotation is some kind of a crazy high school/student party, not anything elegant, like a formal ball - for which 'przyjęcie' would be better. Both can work for a birthday party: przyjęcie urodzinowe, impreza urodzinowa. And true, 'impreza' does mean an event, but I'm not sure if it's used that much in that meaning.
"Impreza" is one of acceptable meanings, but "przyjęcie" is the first meaning given by all the dictionaries, though it is not accepted here. In this course some people may get Polish to English translation rather than the other way around and "impreza" should not be translated into just a party.
Sorry, I should have answered in English, here it goes: "Jest dobra impreza” would be "There is a good party." Here we already know it exists and are expressing our opinion: "It is a good party." The entire Polish sentence translates literally word by word and has the same meaning.
"Ma" is used in the negative ("Nie ma") to indicate that there is "no something". "There is no good party." -> "Nie ma dobrej imprezy.". In the positive case, "ma" used optionally with some subject means "has". "[He] has a good party". -> "[On] ma dobrą imprezę." Note the changed case endings for "nie ma" (genitive) and "[on] ma" (accusative).
These are my understandings; I hope they are right.
They are, although "On ma dobrą imprezę" does not make much sense to me semantically. Generally the only situation where I'd use "mieć" with "impreza" is when it means "to have (planned)".
"Nie możemy przyjechać do ciebie w sobotę, mamy imprezę urodzinową Marty" (We can't come to your place Saturday, we have Marta's birthday party).