can could may

Possibility

'Can', 'could', and 'may' are used to talk about possibility, but they are different in meaning:

• It can be dangerous to travel around here. (a strong possibility based on a general truth or known fact)
• It could/may be dangerous to travel around here. (a weak possibility)

 

 

Permission

'Can', 'could', and 'may' are also used to ask for permission as well as give permission, but not could:

• Can I ask you a favor? (informal)
• Could I have my bill, please? (more formal and polite)
• May I have my bill, please? (even more formal and polite)

 

 

Requests

'Can' or 'could' is used to make requests: 

• Can you take a message?
• Could you take a message? (more polite)

 

 

Future possibility: typical errors 

'Could' is used to express future possibility, but not 'can'. To convey the idea that a future event will possibly not take place, however, use 'may not' or 'might not'. Do not use 'could not':

• The temperature is dropping. It could snow during the night.
• The temperature is dropping. It may/might not snow tonight after all.