上述三个形容词均有“有能力的”或“有资格的”之含义。able多指干某事的才能或能力，如able to make payment，且有形容人多才多艺的含义。capable常用来指满足一般要求的能力，以及解决具体和实际问题的能力[1]。competent和qualified 多强调满足特殊规定或要求的能力；其中，qualified常指从事某一职业或干某事所需的诸如学历、证明等资格，如a qualified voter。而competent则有“能干”的含义，指是否有充分满足某种要求或胜任某项职务或工作的能力。因而，a qualified lawyer 有时并非一定是 a competent lawyer [2]。
[1] "Able suggests versatility and resourcefulness and capable a practical, problem-solving approach." Cf. The Editors of the Readers Digest, Use the Right Word, at p. 106， The Reader's Digest Association Proprietary Ltd. (1971 ).
[2] "Qualified stresses the possession of required skill and is generally applied to professions or trades for which a minimum of schooling or training is required. A qualified teacher has completed the academic training prescribed, but is not necessary competent. ” Id. at p. 106.