"Have a good evening Esther"
Translation:Jioni njema Esther
Ok. The root of both mwema and njema is -ema. You add different prefixes to it depending on the noun class of what it's describing, it's describing jioni (evening). Jioni is in the N/N class, so it must take the prefix belonging to that. Here is a table of what prefixes to add:
Mw- is just another suffix from another noun class to add onto the adjective -ema.
If you have any more questions feel free to reply.
This is a whole phrase, should be learned as a whole phrase. While it might be useful to tear these sentences apart for the grammar, when learning what to say when greeting people it's much better, in my opinion, to learn them as phrases rather than individual words. The dictionary hints should include a translation for the whole phrase.