I can play basketball.
否定句：I can't play basketball.
一般疑问句：Can I play basketball?
肯定回答：Yes,l can.
否定句答：No,l can't.
David can speak English well.
否定句：David can't speak English well.
一般疑问句：Can David speak English well?
肯定回答：Yes,he can.
否定句答：No,he can't.