able 最常用词，多用于褒义，着重指多方面的，经常性的，或潜在性的能力。有时也指高超、非凡的能力。
capable 语气弱于able，指有能力、有潜力或有条件做某事，常与of连用。
competent 强调具有能胜任某项工作的足够技能或其它所需的条件。
参考例句
1. She is my most capable student. 她是我最有才能的学生。
2. I have a capable mother. 我有一位能干的母亲。
3. The situation is capable of improvement. 这种情况是可以改善的。
英英解释
adj.
1. (usually followed by `of') having capacity or ability
2. possibly accepting or permitting
3. (followed by `of') having the temperament or inclination for
4. having the requisite qualities for
5. have the skills and qualifications to do things well
