名詞 pay back:

1. financial return or reward (especially returns equal to the initial investment)
   同義詞：payback
2. the act of taking revenge (harming someone in retaliation for something harmful that they have done) especially in the next life
   同義詞：vengeance, retribution, payback

動詞 pay back:

1. act or give recompense in recognition of someone's behavior or actions
   同義詞：reward, repay
2. take vengeance on or get even
   同義詞：pay off, get, fix

1. He will pay back the money in monthly instalment.
   他將按月分期付款的方式償還。
2. He will pay back the money in monthly installment.
   他將用每月分期付款方式將錢付還。
3. I lent him some money last week but he forgot to pay back.
   上周我借了一些錢給他，但他忘記歸還。
4. Pay back the5,000 Yuan I loan you by today.
   我借給你的5,000元，今天還我吧。