faithful、loyal、constant与true这些形容词均含有“忠实的”之意。
faithful : 多指对人对事或对诺言、誓言的始终不渝，侧重在任何情况下都绝不改变。
- a faithful friend
忠诚的朋友 - They are faithful supporters of the Labour Party.
他们是工党忠实的拥护者。 - His faithful old dog accompanied him everywhere he went.
无论去哪里，他那条忠诚的老狗总是伴他左右。
loyal : 一般指对祖国、领袖的忠诚或对事业、原则、誓言的忠实，不动摇。
- Jack has been a loyal worker in this company for almost 50 years.
杰克是这家公司忠诚的员工，已经干了快50年了。 - When all her other friends deserted her, Steve remained loyal.
当其他朋友都离她而去的时候，史蒂夫依然对她不离不弃。
constant : 侧重指思想状态的稳定或信念的坚定，也指爱情或感情等的忠贞不渝。
- Fred had been her constant companion for the last six years of her life.
在她生命的最后6年，弗莱德一直是她忠实的伴侣。
true : 侧重个人感情上的忠实和不动摇。
- She has vowed to remain true to the president whatever happens.
她已经宣誓在任何情况下都将效忠于总统。 - He said he'd repay the money the next day, and true to his word (= as he had promised), he gave it all back to me the following morning.
他说第二天还钱，果然说到做到，第二天上午就把钱全部还给了我。
