The money I borrowed from him was repaid with interest.

　　What he did was just to protect his own interests.

　　interest在教材中的意思为兴趣和使感兴趣,分别作名词和动词用。

　　上面句子中的两个interest都作名词用。作利息讲时,为不可数名词;作利益,好处讲时,多用复数形式。

　　第一句可译为我向他借的那笔钱是带息偿还的。;第二句可译为他所做的一切仅仅是为了保护他自己的利益。