"I think I will probably go to the party tomorrow."
"私は多分明日のパーティーに行くと思います" was marked wrong, is it? I think the given solution sounds more like you plan to just look for some random party to join tomorrow, while the question sounds more like there is a planned party tomorrow that you are contemplating. Tl;dr: Question says "THE party", solution sounds more like "A party".
The official answer above, has a meaning of, "tomorrow, I will go to a party, I think." Your version, although it has almost the same meaning, is "I think I will go to tomorrow's party". I'm not a native speaker, but it feels like your answer implies there's a party everyday...
should 明日は多分パーティに行くのは思います。be accepted or is the nuance different? Would I need to put in a 私が after the のは to make that construction work?
You need to understand the plain form for each part of speech.
- Noun + だ => e.g. 学校です -> 学校だ -> 学校だと思います
- Verb (dictionary form) => e.g. 食べます -> 食べる -> 食べると思います
- i-adjective (dictionary form) => e.g. 高いです -> 高い -> 高いと思います
- na-adjective + だ => e.g. きれいです -> きれいだ -> きれいだと思います
Nope, as far as I remember the only difference was that I didn't actually use the comma in my answer since I used the word bank options (Duolingo ignores them anyway).
I did report it, but I'd be surprised if it was already added. Maybe I was stupid and accidentally tapped words in the wrong order, don't have a screenshot to check.
Btw. where can I see this list of all correct solutions?