“另外一回事”常见的英文表达有：
a different matter/story/kettle of fish：这几个短语意思相近，都用于表示与之前提到的情况不同，是另一码事。例如：
This may seem similar, but it's actually a different matter.（这看起来可能相似，但实际上却是另外一回事。）
Winning a local competition is one thing, but representing your country at the Olympics is a different story.（赢得地方比赛是一回事，但在奥运会上代表国家参赛则是另外一回事。）
I thought fixing the leak would be easy, but dealing with the water damage is a whole different kettle of fish.（我本以为修好漏水处很容易，但处理水渍就是另外一回事了。）
another thing altogether：强调与之前所谈完全不同，程度更深。例如：
He might be good at sports, but being a good leader is another thing altogether.（他可能擅长运动，但成为一个好的领导者则是另外一回事。）