We use the modal can to make general statements about what is possible: It can be very cold in winter.
We use the negative can’t or cannot to show that something is impossible: That can’t be true.
We use can to talk about someone’s skill or general abilities: She can speak several languages.
We use can to ask for permission to do something: Can I ask a question, please?
We use can to give permission: You can go home now if you like.
We use can I … to make offers: Can I help you?