B2. We use 'can' to ask for permission to do something. (e.g. Can I ask a question, please?)
a. 'could' is more formal and polite than 'can'. (e.g. Could I ask a question please? Could we go home now?)
b. use 'can' to give permission. (e.g. You can go home now if you like.) Also we use 'can' to say that someone has permission to do something. (e.g. We can go out whenever we want.)
B3. Instructions and requests: We use 'could' you and as a polite way of telling or asking someone to do something. (e.g. Could you take a message please?)
a. 'can' is less polite. (e.g. Can you take a message please?)
B4. Offers and invitations: We use 'can I ~' to make offers. (e.g. Can I help you?)
a. We sometimes say I can ~ or I could ~ to make an offer. (e.g. I can do that for you if you like.)
Idiom 365
Idle Hands Are The Devil's Tools: have nothing to do are more likely to get into trouble or commit a crime
Example: The old person taught me that children should always be kept busy, if not, Idle hands are the devil's tools!