Thank all of you for your explanations

You're thinking of "obligatory" and "compulsory". In some cases, you can use either one. Of course, "compulsory" means you have no other option. "Obligatory" is closer to "you really, really should". It's a rule of law. As the other members have explained, "obligate" has a somewhat different sense.

Hi Nat, compulsory = 必定要 obligated = 有责任要 For more English ideas... 你可以看看英語亞洲 www.englishasiaonline.net :)

Close, but they are not the same. "To be obligated" to do something means that *you* feel that you need to do it (because it's right thing to do, because someone did you a favor and you feel that you should reciprocate). It's your choice. "Compulsory" is mandatory. You *have to* do it. It's not your choice.