I was going to ask why "the party" not "a party", but it looks like someone answered that two years ago. So now my question is ... when was it explained that there is a difference between 'la mesibah' and 'le mesibah'? The grammar hints, by the way, actually give both 'the party' and 'a party' as possible answers.
I don't know if or where Duolingo explains that adding ה to certain word-particle combinations only changes the vowel, but... Duolingo doesn't have a lot of in-depth explanations, to be honest. It works best as practice/supplement to a more thorough and organized course of study, rather than as one's entire study on its own.