CAN:
1. (+) I can play volleyball
( - ) I can't play volleyball
(?) Can you play volleyball?
2. (+) I can tell you about it
( - ) I can't tell you about it
(?) Can you tell me about it?
3. (+) She can do aerobics
( - ) She can't do aerobics
(?) Can she do aerobics?
COULD:
1. (+) It could be dangerous to cycle in the city
( - ) It couldn't be dangerous to cycle in the city
(?) Could it be dangerous to cycle in the city?
2. (+) I could understand what she said
( - ) I couldn't understand what she said
(?) Could you understand what she said?
3. (+) My grandfather could speak five languages
( - ) My grandfather couldn't speak five languages
(?) Could your grandfather speak five languages?