"Neng" (能) and "Hui" (會) are both auxiliary verbs denoting ability, and are often translated as "can" in English. "Neng" refers to ability conditioned by situation or physical capacity. "Hui" refers to ability conditioned by having learned a specific skill (i.e. speaking a language, walking, riding a bike etc.). While you might know how ("hui") to ride a bike, you might not have the capacity ("bu neng") to ride for 50 miles, or the situation (you have homework to do for example) might preclude you from taking a ride at a certain time (again "bu neng".) Practice making distinctions below