"Have a good night Rehema"
Translation:Usiku mwema Rehema
As a rule of thumb: "mwema" is for animate (plants, humans, animals) and nouns beginning with an m- or u-. (pluarl: "wema") "njema" is for nouns from the N-class (9/10) - they're a lot; as other forms are rare (I don't think we've learned any in this course so far, you can stick with that).
Night is not an animate object - "mwema" also is used for abstract nouns starting with "u-" (such as "usiku" - night) and plants singular (such as "mti" - tree) morning and afternoon (I'm not sure about evening) are Arabic loan word, which thereby fall into class 9/10; usiku, however, would be a Bantu word and has always been in class 11(/10) - the Bantu words formerly used for morning, afternoon etc. have been overtaken by the Arabic (meaning, they weren't always in that class). Loan words are usually put into the "ma-" class (6) or "i-"/"zi-" (9/10).
It depends on the noun class. There is upto 19 classes both singular and plural and it is usually divied in inanimate objects, living things, nature. Anyway my problem with is part is that have a goodnight should be translated to kuwa na usiku mwema. Fluent speak, just doing some revision