能
néng
Expressing ability or possibility with "neng"
能 is "can" as in circumstances allow — you're free, it's physically possible, you're permitted.
能 + Verb
How it works
我今天不能来 means something is stopping me today, not that I lack the ability. 你能帮我吗? asks whether you're in a position to help.
The contrast with 会 is about the source of the ability: 会 comes from learning, 能 from circumstances. 我会游泳，但今天不能游 — I can swim, but I can't today.
能 also covers capacity: 他能喝十瓶啤酒.
Examples
zhōng wén zhè me nán, nǐ néng xué hǎo ma?
中文这么难，你能学好吗？
Chinese is so hard — can you really master it?
wǒ jīn tiān bù néng lái
我今天不能来。
I can't come today.
nǐ néng bāng wǒ yí xià ma?
你能帮我一下吗？
Could you give me a hand?