can

If someone can do something, they are able to do it.

Example: Most birds can fly.

If someone can do something, they are allowed to do it.

Example: You can't smoke here.

You use can to ask somebody to do something (if you know the person well).

Example: Can you open the door for me, please?

If something can happen, it is possible.

Example: It's so small that it can't be seen.

If someone tells you something can't happen, they think it's not a good idea.

Example: You just can't keep smoking.