Duolingo describes a "plural suffix" -ni that gets appends to "some words".
There are certain words, like karibu, asante, shikamoo, or kwa heri that, when used to address more than one person, take the suffix -ni to indicate plural references. So, saying asante "thank you" to a group of people would be asanteni.
I'm trying to make this more memorable than applying to "some words." Does the -ni suffix correspond to some linguistic category that I can look up?
This question was originally asked on Language Learning.