"Have a good morning"
Generally: You only use "mwema" for people/animals (living beings) and words beginning with m- or u-. "njema" is for class 9/10 (N-class) only, but it's a large class and has very few nouns only with m-/to whom the above would apply. (There are other forms as there are 16/18 classes; or half - depending on the classification system one uses :) ) These two are the most common - hope that helped!
So Swahili's noun classes are keys to adjectives and verbs - here's a table: https://en.wikipedia.org/wiki/Swahili_language#Agreement And more description: https://en.wiktionary.org/wiki/Appendix:Swahili_noun_classes It takes some time to work it out and get into. Sorry :)