What linguistic category does Swahili's *-ni* correspond to?

Question

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.

Answer 1

It is a plural imperative inflectional suffix added to verbs, which can also be added to the above words, which are not verbs, but are interjections meaning literally "near" (welcome), "thanks", "grab the feet" (a respectful greeting) and "bye-bye", thus "Welcome, y'all". Verbal examples are toka "go away!", tokeni "go way (pl.)!".

Answer 2

I wish I had a more straight forward answer to give, but "ni" is simply a morpheme that can have many different meanings depending on how it is used

• it can show plural when added to a verb to show that many people are being addressed
• when added to a noun it can change it into an adverb of place

among many other more complex to explain concepts