pay off，讀音：英[pe f],美[pe f]。
vt. ①付清，還清（債務）。
②取得成功。
③賄賂。
④報復。
vi. 得到好結果。
例句：
①He paid off the debt of his friend with the money raised by his writings.
他用稿費的收入償清了朋友的債務。
②Walter will be paid off for his negligence.
華爾特將因他的失職而受到處罰。
③It has been rather costly to install the machinery, but it should pay off in the long run.
安裝這套機械代價很高，但從長遠來看它會賺錢的。
pay back，讀音：[pe? b?k]
vt. 償還；報答。
例句：
①He paid the money back promptly.
他很快就把錢還了。
②I'll pay her back for her hospitality by inviting her to dinner.
我要請她吃飯以回報她的殷勤款待。