上述三個形容詞均有“有能力的”或“有資格的”之含義。able多指干某事的才能或能力，如able to make payment，且有形容人多才多藝的含義。capable常用來指滿足一般要求的能力，以及解決具體和實際問題的能力[1]。competent和qualified 多強調(diào)滿足特殊規(guī)定或要求的能力；其中，qualified常指從事某一職業(yè)或干某事所需的諸如學歷、證明等資格，如a qualified voter。而competent則有“能干”的含義，指是否有充分滿足某種要求或勝任某項職務或工作的能力。因而，a qualified lawyer 有時并非一定是 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.