On grammatical class:

ca'a
modal aspect: actuality/ongoing event.
ka'e
modal aspect: innate capability; possibly unrealized.
nu'o
modal aspect: can but has not; unrealized potential.
pu'i
modal aspect: can and has; demonstrated potential.
na'epu'i
modal aspect: has never, and perhaps cannot; other-than-demonstrated-ability.
su'i'u
modal aspect: members do the reciprocal activity