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)