"Have a good night Emilian"
Translation:Usiku mwema Emilian
Those are noun class prefixes (the equivalent of gender in Romance languages). Njema is used with siku and jioni because they are N-/N- nouns. Mwema is used with usiku because it is a U-/N- noun, and with mchana because it's a M-/MI- noun. It seems like a lot to remember right now, but you'll get used to identifying the different noun classes as you go along.
Thanks for your reply which I now understand as I have learned a little about noun classes. I am really enjoying this Duolingo Kiswahili course, but it is not quite good enough in the way it presents grammer and vocabulary in the exercises before it has been explained or introduced. I suppose that it because it is a beta version. Still I am surprised and pleased at how far I have got in three weeks.
What is an N- noun? And what is a U- noun? I dont understand as that letter isnt within all of the nouns used here?
Siku means "day". You could say Usiku mwema Emilian, but note that you then have to change the prefix on -ema to match the class of usiku.