​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。