Do you know if there are any hints at decoding what the noun class of a word is?
If you go to the wikipedia page at https://en.wikipedia.org/wiki/Swahili_language and go to section 5.3- Agreement. You can see how the -a preposition (of) changes depending on the noun class.
It's one of the more advanced things so if you get it wrong, native speakers will still understand what you're saying.
Or how is your evening. This is a greeting, isn't it. How's your morning/day/evening going = Good morning/day/evening.
The proper sense is missing from the dropdown glossary.
I did, it makes sense, but I just don't know if its considered an idiom
It is an idiom. An idiom is just something that has a meaning other than the literal interpretation.
good night would be 'usiku mwema', this literally is asking how the night is. Good evening would be 'habari za jioni'