It would be helpful if the lessons words were translated at the beginning of tne lesson. Also, when the noun-classes or other things change it would be helpful if there was a choice with an explanation.
Why not njema?
Mchana is a MI-class noun. Adjectives modifying singular MI-class nouns take a m- prefix (or mw- before a vowel). Hence, "mwema." (https://en.wiktionary.org/wiki/mchana)
The adjective -ema means "good". Depending on the noun class, different prefixes will be attached to the front. For all the versions of -ema, see this link on Wiktionary.
Can't "-zuri" be used instead of "-ema"?