able，capable和competent都可以表示「有能力的」，均可作定語或表語，但有區別
1. able指人有足夠的智慧或力量，聰明能幹，也可表示人的體力或身體狀況，常用於好事，作表語時後接to do sth.。
the ablest student in the class
班上最有才華的學生
He is an able architect. 他是個很有才幹的建築師。
We aim to help the less able in society to lead an independent life.
我們的宗旨是幫助社會上能力較弱的人獨立生活。
The older child should be able to prepare a simple meal.
那個年紀稍大的孩子應當會做一頓簡單的飯菜。
The patient was poorly able to sit up. 病人僅能勉強坐起來。
You'll be able to read in peace.
你就能安心地讀書了。
注意，able和capable都用來表示某人能做某事。able 既可表示因擁有某種知識或技能而能做某事，也可表示客觀可能：
He wondered if he would be able to climb over the rail.他不知自己能否攀過欄杆。
They were able to use their profits for new investments .他們得以將利潤用於再投資。
要注意的是，如果用過去時，則表示實際(成功)做了該事：
We were able to reduce costs .我們做到了降低成本。
They were able to achieve a settlement without using military force.
他們沒有訴諸武力就解決了問題。
She was able to command the respect of the class.
她贏得了全班的尊敬。
capable既可表示具有做某事的技能或知識，也可表示做某事有較大的可能性:
The workers are perfectly capable of running the organization themselves.
工人完全能夠自己管理這個機構。
She was quite capable of falling asleep.
她很容易睡著。
capable 還可表示具有某種情感或能夠做出某一行為：
He's capable of loyalty.
他能做到忠心耿耿。
Bowman could not believe him capable of murder.
鮑曼無法相信他會殺人。
capable亦用於指汽車、機器等的性能：
The car was capable of 110 miles per hour.
這輛車時速可達110英里。
able或capable形容人時表示「有能力的」、「能力強的」：
He's certainly a capable gardener.= He's certainly an able gardener.
他絕對是一名能幹的園丁。
2. capable表示做某事所必需的能力，側重於有潛能或可能性，可指好事與壞事，語氣比able弱，作表語時後接of（doing）sth.。
指具有接受、適應等能力的，強調足以勝任某事；作表語時常後接介詞 of，表示「能…的」，其主語可以是人或物。
She is a very capable driver.她是個技術熟練的駕駛員。
He is most capable of leadership. 他極有領導才能。
My mother is a strong, capable woman.
我媽媽是個健壯、能幹的女人。
She has become a capable pilot.
她已成為一名有本領的飛行員。
He'll leave the team in your capable hands.
他將把球隊交給你這位能手。
He is capable of doing everything.
他什麼事都能幹。
The hall is capable of holding about three hundred people.
這個大廳能容納大約三百人。
She is perfectly capable of taking care of herself. 她完全能夠照顧自己。
He's quite capable of neglecting his duty. 他很有可能玩忽職守。
That scoundrel is capable of anything. 那個惡棍什麼壞事都幹得出來。
3.competent著重具有勝任某項工作的技能或所需條件。「足以勝任的；有能力的；稱職的，有決定權的」：
Make sure the firm is competent to carry out the work.
要確保這家公司有能力完成這項工作。
He is not competent to the task of teaching English.
他不能勝任英語教學工作。
He is a highly competent teacher.
他是一個十分稱職的教師。
He's very competent in his work.
他非常勝任自己的工作。
I had always considered myself a strong, competent woman.
一直以來，我始終認為自己是個女強人。
The people who will be on the committee are moral, cultured, competent people.
進入委員會的人要品行端正，有文化，有能力。
A competent typist is not necessarily a competent secretary.
合格的打字員不一定就是合格的秘書。
I was deemed to be a competent shorthand typist.
我被視為是一名能幹的速記打字員。
The case was referred to a competent authority.
事情已交給有關當局處理。
He was a loyal, distinguished and very competent civil servant.
他是一位敬業、優秀而且非常稱職的公務員。
Most adults do not feel competent to deal with a medical emergency involving a child.
大部分成年人不懂得如何對兒童實施急救。
The most competent adults are those who know how to do this.
最有能力的成年人是那些知道怎麼做的人。