ability
◆ actual skill, either mental or physical; native or acquired
实际能力，脑力或者体力上的;指人先天的或学来的各种能力。
例：
...the ability to bear hardship.
吃苦耐劳的能力...
Capability
◆ unique fitness for a defined end; sometimes may be used in place of ability, but its use in place of capacity is incorrect
多用于人，针对特定目的的具体能力(常有“胜任”某职务的意思);有时可用来代替ability，但它不能替代capacity
◆ often refers to extremes of ability
通常指你能达到的能力的极限
例：
It was, in any case, beyond the capability of one man.
无论如何，一个人是做不到的。
Capacity
◆ potential to develop a skill, usually mental; native, as opposed to acquired
侧重指潜在能力，通常不指体力，多指才智，尤指接受与领悟能力;先天的，不是后天努力能得到的
◆ a measure of such ability.
衡量某一能力的标准或者尺度;capacity实际上多与数量或容量有关，表示能力的一种度量。通常用在物上，表示某物的容纳等能力
例：
vital capacity n. 肺活量
最后，
我们放在句子里对照理解一下
I have the ability to write.
我能写作。
意思就是：
I've always been interested in writing, and I took some classes.
我一直对写作很感兴趣，我也上过一些写作课。
I don't have the capability to write a novel.
我不能写小说。
意思就是：
My skill is in poetry, not creating a fictional story.
我很擅长写诗歌，但是我不擅长创作小说。
I do have the capacity to write a novel, but I don't want to.
我能写小说，但是我不想。
意思就是：
I don't want to put the mental effort into writing a long story.
我不想把精力花在写一个长篇故事上。
