a different kettle of fish
/ə ˌdɪf.rənt ˌket.əl əv ˈfɪʃ/idiom
a completely different matter or situation
совсем другое дело
Русский аналог: «совсем другое дело; две большие разницы»
Часто: that’s a different kettle of fish. Не путайте с a fine / pretty kettle of fish — «неприятная ситуация».
“Cooking for two is easy, but cooking for fifty is a different kettle of fish.”
“I can speak English, but writing a legal contract is a different kettle of fish.”
“Teaching teenagers is a different kettle of fish altogether.”