Expressing "to repay" with 报答 and 回报
The verbs 报答 and 回报 are the same in translation, but different in usage.
The verb 报答
The verb 报答 
indicates that a person wants to express his gratitude to another person by doing something.
indicates that a person wants to express his gratitude to another person by doing something.Examples
我希望有一天能为你做点什么来报答你
I hope that one day I can do something for you to repay you
wǒ xīwàng yǒu yì tiān néng wéi nǐ zuò diǎn shénme lái bàodá nǐ
我怎么也报答不了您的恩情
I am deeply indebted to you for your kindness
wǒ zěnme yě bàodá bu liǎo nín de ēnqíng
我欠他们很多，但我不知道如何报答他们
I owe them a lot, but I don't know how to repay them
wǒ qiàn tāmen hěn duō, dàn wǒ bù zhīdao rúhé bàodá tāmen
The verb 回报
The verb 回报 can be used in both positive and negative connotations.
With a positive connotation, it indicates that a person wants to express his gratitude by doing something or paying with money.
It can also indicate that an action will pay off handsomely in the future.
can be used in both positive and negative connotations.
With a positive connotation, it indicates that a person wants to express his gratitude by doing something or paying with money.
It can also indicate that an action will pay off handsomely in the future.Examples
这是本难读的书，不过它会丰厚回报为之所付出的努力
This book is hard to read, but the effort will pay off handsomely
zhè shì běn nán dú de shū, búguò tā huì fēnghòu huíbào wéi zhī suǒ fùchū de nǔlì
现在是回报祖国的时候
It's time to fulfill our duty to our country
xiànzài shì huíbào zǔguó de shíhòu
With a negative connotation, 回报 means "to retaliate," "to repay in kind".
Examples
对这种奸诈应以同样的方式回报
We should retaliate against them for this betrayal
duì zhè zhǒng jiānzhà yīng yǐ tóngyàng de fāngshì huíbào
回报仇敌
To retaliate against the enemy
huíbào chóudí
回报 as a noun
回报 can also be used as a noun "retaliation".
Examples
这家公司正开始收获长期投资的回报
This company is beginning to reap the benefits of its long-term investments
tā de nǔlì dédào le fēnghòu de huíbào
似乎他们应该给我们点什么作为回报才像话
It seems like they should give us something in return
sìhū tāmen yīnggāi gěi wǒmen diǎn shénme zuòwéi huíbào cái xiànghuà
他的努力得到了丰厚的回报
His efforts were handsomely rewarded
tā de nǔlì dédào le fēnghòu de huíbào