The pillow is long (?)
Is this different from saying: a long river?
I believe so because "ni" means "is" and acts as the verb to complete the sentenece. "A long river" is just a clause missing its verb.
The slowed pronounciation was nit working
How would I say "The river is deep"?
I found this translation for 'deep rivers': mito yenye kina kirefu, which literary means 'rivers with tall depth' or 'rivers having a tall depth'. So, my guess is that 'the river is deep' then would be mto una kina kirefu.