There's no difference to me so far. sinds i've been in tanzania ive learned it to be very simple: Asubuhi njema Mchana mwema Jioni njema Usiku mwema There's no difference in the meaning between njema or mwema only that they are bound to the time. So the example here above is how you"ll always be correct. I dont know the function of these differences but maby one day i will
It's linked to this thing called noun class which I'm ill equipped to explain, however it is important in Swahili. Every noun belongs to one of the noun classes and depending on which one indicates how the adjective agrees.... Lots of other posts by people helpfully explaining this in different places...