The modal auxiliaries can and could have several uses. They are used to talk about ability and possibility, to ask for and give permission, and to make requests and offers.
Examples are given below.
- She can speak English. (This shows her ability.)
- He could solve the puzzle in just two minutes. (Ability)
- She could speak when she was just two. (Ability)
Use cannot and could not to talk about inability.
- I can’t speak French.
- They could not open the door.
Note that cannot is written as one word.
- She can come in the evening. (Possibility)
- Glass can be blown. (Possibility)
- There could be a strike next week. (Weak possibility)
- It could rain this afternoon. (Weak possibility)
- Can I watch TV after doing my homework? (May is considered more correct here, but ‘can’ is gaining acceptance.)
- You can go home early today. (Permission)
Offers and requests
- Could you help me? (Request)
- Can you post this letter for me? (Request)
- Could I have some more tea? (Request)
- Can I help you? (Offer)