1. able, capable, competent

able为常用词，指具有做某事所需的力量，技巧，知识与时间等，一般下效率无关，用作定语表示能力超出平均水平。如：A cat is able to see in the dark.

capable 指满足一般要求的能力，可以是表现出来的，也可是潜在的，搭配是be capable of +doing。用作定语，表示的能力没有able表示的能力强。如：He is capable of running a mile in a minute. He is a very capable doctor.

competent 指“胜任”，“合格”，或受过专业技术等训练的，但不是超群的能力。如：A doctor should be competent to treat many diseases.

2. aboard, abroad, board, broad

aboard 在船(或飞机，车)上。如：I never went aboard a ship.

abroad 副词，在国外或海外。如：He often goes abroad.

board 为动词，上(船，飞机，车)。如：The passengers are boarding the plane now.

broad 为形容词，宽广的。如：He has very broad shoulders.