ability
actual skill, either mental or physical; native or acquired
實際能力，腦力或者體力上的；指人先天的或學來的各種能力。
例：
...the ability to bear hardship.
吃苦耐勞的能力...
Capability
unique fitness for a defined end; sometimes may be used in place of ability, but its use in place of capacity is incorrect
多用於人，針對特定目的的具體能力（常有「勝任」某職務的意思）；有時可用來代替ability，但它不能替代capacity
often refers to extremes of ability
通常指你能達到的能力的極限
例：
It was, in any case, beyond the capability of one man.
無論如何，一個人是做不到的。
Capacity
potential to develop a skill, usually mental; native, as opposed to acquired
側重指潛在能力，通常不指體力，多指才智，尤指接受與領悟能力；先天的，不是後天努力能得到的
a measure of such ability.
衡量某一能力的標準或者尺度；capacity實際上多與數量或容量有關，表示能力的一種度量。通常用在物上，表示某物的容納等能力
例：
vital capacity n. 肺活量
最後，
我們放在句子裡對照理解一下
I have the ability to write.
我能寫作。
意思就是：
I've always been interested in writing, and I took some classes.
我一直對寫作很感興趣，我也上過一些寫作課。
I don't have the capability to write a novel.
我不能寫小說。
意思就是：
My skill is in poetry, not creating a fictional story.
我很擅長寫詩歌，但是我不擅長創作小說。
I do have the capacity to write a novel, but I don't want to.
我能寫小說，但是我不想。
意思就是：
I don't want to put the mental effort into writing a long story.
我不想把精力花在寫一個長篇故事上。
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