> When is bhaasati (he says, he speaks) used, and when is vadati (he
> says)used.

Generally I think vadati is used for "say", and bhaasati for
"speak/talk":

sati~nca khvaha.m bhikkhave sabbatthika.m vadaami

"Mindfulness, I say, o Bhikkhus, is ever useful." (S. 5.2.3)

manasaa ce padu.t.thena bhaasati vaa karoti vaa

"if, with a mind that is corrupt, he speaks or acts" (Dhp. v. 1)

> Also the same for bhavati (he is, there exists) and hoti (he is,
> there
> is). I can not find reference as to how to know which and/or when to
> use, i.e which kind of sentence is one or the other used in?

See Warder, p.12. But bhavati seems to often imply "becomes" rather
than "is".

bhuu+ati=bhavati
huu+ati=hoti

bhavatu sabba ma"ngala.m,

Yuttadhammo