be another matter/thing idiom
另当别论；情况不同
to be a different situation that is likely to be judged differently
Cars are useful, but their impact on the environment is another matter altogether. 汽车很有用，但对环境的影响则应另当别论。
另外一回事；真正难做的事情
to be very different and likely to involve more problems or difficulties
Wanting to help the homeless is one thing, finding cheap secure accommodation for them is quite another (thing). 希望帮助无家可归的人是一回事，而为他们找到廉价安全的住所就难了。