"It is a good party."
Translation:To jest dobra impreza.
"To jest dobra impreza" is an incorrect translation. The Polish word "impreza" has a much broader meaning than a party = "przyjęcie." It could also mean an organized concert or a formal ball, or many other entertainment events. In fact "impreza" = event.
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.
Well, technically... added.
Note that in this particular sentence, if we don't have a politics context, the sentence will be understood totally differently. "dobra partia" is a "good match" or maybe even more often "good catch". So either someone who is a suitable partner, or even someone who is 'worth dating/marrying'. Like a handsome, rich bachelor from Jane Austen's novel.