goes before a verb and means "can" in the sense that circumstances allow it: you are free, well enough, permitted or physically able. It is negated with 不.
？
Could you speak a bit more slowly?
is not the everyday negative of 能. It reports that someone didn't manage to do something.
。
He didn't manage to come yesterday.
能 or 会
is "can" for a skill you have learned; 能 is "can" for whether the situation lets you.
，。
I can drive, but I can't today.
Swap them and the sentence changes: 我不会开车 says you never learned, 我不能开车 says something is stopping you.
Permission
In a request, 能 and overlap: 我能在这儿坐吗 and 我可以在这儿坐吗 both ask "Can I sit here?".
In the negative, 不能 says something is not allowed.
。
You can't eat in the library.
Capacity
能 also states how much someone is capable of, usually with a number.
。
He can drink five bottles of beer.
。
He can walk twenty kilometres in a day.
Lingaku