Origin of the epistemic in Vougerå
In Proto-Sumric there was the word *spḿf- “to predict”. In Old Tuura it became the verb sbo͂fti “to predict”. In Old Tuura, its inflected form coud be inserted at the end of a clause to show that the subject believes that the verb (inflected for the future tense) will happen, often the clause will subordinated e.g:
nam donåṡṡyd jåguovüüssuod, sbo͂ffåṡ
king make-3S.ANIM-FUT book-ACC.INAN=SUB, predict-3S
“That the king will write a book, he predicts”
sulṡmmyd samwasåssuod, sbo͂fṡm
see-1S-FUT moth-ACC.INAN=SUB, predict-1S
“That I will see a moth, I predict”
By Late Old Tuura the verb “predict” had become more of a discourse particle. It no longer agreed with the subject of the clause and now only inflected for the 1st person. e.g:
nam donåṡṡyd jåguovüüssuod, sbo͂fṡm
king make-3S.ANIM-FUT book-ACC.INAN=SUB, predict-1S
“That the king will write a book, I predict”
sulṡmmyd samwasåssuod, sbo͂fṡm
see-1S-FUT moth-ACC.INAN=SUB, predict-1S
“That I will see a moth, I predict”
In Soura, Old Tuura’s daughter, the discourse particle had become the deontic modal particle sbom to express that it is important for the verb to be carried out, or to insinuate expectations. The verb need not be in the future tense, now it will usually appear in the present but it can be used with other tenses with the deontic, neither does the verb phrase need to be subordinated anymore.
“the king should write a book”
In Vougerå, Soura’s daughter, the deontic modal particle had become the epistemic suffix -sbu which attaches to verbs to insinutate that the verb must be carried out.
see-1S-EPIS moth-ACC.INAN
“I must/have to see a moth
It also became an imperative in the “royal” register when speaking to the king. The “royal imperative” is formed with subject being refered to in the third person (the indirectness is seen as more polite). It is considered rude to use the usual imperative (discussed here) with the king.
“write a book” (addressed to the king)
Over time the roya register came to be used with anyone of a higher social status than the speaker, so a man wi use it with his gran, his gran would use it with the local chieftan, the local chieftan would use it with the king. The royal register is mostly unidirectional in that no two speakers use it with each other. If two speakers are on the same social standing (such as two chieftans) then they use the normal register, which of course no one uses with the king.