A is one thing, B is another

Từ điển Oxford Advanced Learner 8th

A is ˈone thing, B is aˈnother | it's ˈone thing to do A, it's aˈnother thing to do B idiom
B is very different from A, for example it is more difficult, serious or important
•Romance is one thing, marriage is quite another.
•It's one thing to tease your sister, but it's another to hit her.
Main entry: ↑thing_idiom