well, "neng" means ability, able to do something,capable of doing sth, qulified to do sth.
as following:
i can (neng) pass the english exam , he can (neng) do that job
"hui "means promise, will , threat , and possiblities ,sure. as following:
i will(hui) lend u the car as i promised (promise)
i will(hui) kill u if u dont pay me salaries(threat)
he is likely to come back for dinner. its sure (hui) to rain . (possiblities.)
sometimes they can both be used to mean "can", having various skills
he can play piano. (u can use neng,or hui, either will do)