I was wondering whether those “Nice guess! But think more like…” messages mean that the answer I gave could also work or is it more “we see you were just trying to guess the answer but failed”?
In other words, can I consider what I wrote a viable alternative, but it’s just not what Bunpro is looking for in this example, or was I way off and should never use that again?
Yes they’re generally fine based on the English translation given. Occasionally they are mistakes but if so it explicitly says it, for example “you’re missing a particle!”
Most of those were added because it’s weird, frustrating, and potentially confusing to be marked entirely wrong if you’ve attempted to complete the sentence with different (acceptable) Japanese phrasing. The messages are meant to reassure you that you did provide valid Japanese, but something else is being tested in that review.
If your guess is wrong but you’ve made a common mistake, the message should elaborate to let you know what it was. (In these cases, I’d recommend failing the review instead of fixing it so that you can later pass it without needing the warning, but that’s ultimately left to the user’s discretion.)
I agree that sometimes these “Nice guess! But think more like…” things could definitely be rewritten, and are a bit confusing.
I think the initial motive was that it is a ‘nice’ way to say that you’re not 100% correct, but on the user side you can often feel like ‘I just want to know if I am ‘right’, or ‘wrong’, then you can tell me to try an alternative’.
I have made a note of this, and will think about some possible ‘explicit’ alternatives that we could potentially use.
This is an idea I had a long time ago, but I think it’s worth mentioning again:
Have the answer field flash a GREEN warning when the user’s attempted answer is perfectly valid, but is not the grammar being tested— have it flash YELLOW/ORANGE warnings (whichever color is currently being used) to indicate minor/common grammatical errors & unnatural/undesirable nuances.
Maybe nuance differences should be marked with green, though? Or even have their own color. Hmm… 
Lastly, to ensure that this functionality would be useful for everyone (including colorblind users), maybe flash a ◯ or △ for the green warnings, and a 
or × for the yellow/orange warnings (where something is actually, genuinely, incorrect).
Edit:
After a bit more thinking, maybe this would be better:
I think the ◯ (maru) and × (batsu) idea is really good! Especially if the meaning of it is introduced to users straight away, so that they become accustomed to it. It also reflects the Japanese system of marking papers, which is kinda cool!
The triangle from your edit is also nice. I think all of them could see potential use! It may even be worth doing all of them in the same ‘high contrast’ color. As the colorblind point may make certain colors difficult to see on the dark mode background.
Just to be clear, I wasn’t complaining 
I’m glad not every single little slip-up is immediately marked wrong, it helps you get a much better understanding of the language’s structure and nuance. I just wanted to make sure I was drawing the right conclusions from the messaging.
I do like the idea of clearly differentiating between “everything’s correct, just not what we’re looking for” and “you’re not entirely right”. As I progressed, I would get the first type increasingly more often, which I found encouraging, as it meant I wasn’t just parroting the answers provided but was actually understanding how the language worked, since I used a grammar point from a different example correctly in a new context. If we can make that even clearer to the user, I’m all for it.
I always thought those messages were one of the best features of Bunpro and that they are incredibly useful to learn about different ways to say something or in some cases to learn why something doesn’t work in a given situation.
I even got into the habit to undo correct answers to see if other grammar points I can think of will get me a message, or even try to use some of the related grammar points.
So if there were some kind of redesign to happen to those messages, I would love to be able to cycle through or maybe to see an extra tab with the other solutions that would be possible (but Bunpro isn’t looking for here) and maybe also the wrong answers that get you an explantion. I think there is so much that can be learnt from those messages and that it’s a waste to just see them when you use something else than the main answer.