English (or Japanese, hehe) isn’t my first language, so I might miss some nuances. However this surprised me:
I wonder, is there some reason, why this wasn’t accepted? Sadly I don’t really see significant differences in meanings.
Can I simply add discussion to my synonyms?
You can add anything you want as a synonym provided it isn’t blacklisted. So, yes, you can add ‘discussion’ and it will be accepted as correct in future reviews.
Granted as a non-native English speaker you, understandably, didn’t see a significant difference in the meanings, but as @trunklayer pointed out, there is a difference! So, in the future (and in this instance as well), you need to decide for yourself whether to use a synonym to make things easier for you or to investigate the meaning of the synonym that you want to use and determine if it is appropriate or not.
Best of luck to you 
regardless of what you decide!
Thanks, it a nice breakdown and my English level is sufficient to feel the difference. I’ll mark the thread as solved.
As for my original question: one dimension is proper English translation, the other dimension is what the Japanese word actually covers. That’s pretty important for my Japanese studies (not that I wouldn’t learn new English words and/or improve some spelling while studying WaniKani). While checking the related threads at the bottom of the page I found some related vocabulary info and I’ll cross-link it here. It might help when someone else finds themselves in similar situation as I was.
