Como dizer "Uma coisa é...Outra coisa é..." em inglês

Primeiro! Tal uso dessa expressão no N.1 e 2 é considerado errado para o uso-costume do estado de SP, ou, à caso de raridade ou de equívoco, nunca ouvi tal uso ( de simplesmente distinguir A de B ) que soa estranhamente errado para mim

Agora, acredito que sirva falar desta forma, se entendi corretamente o que deseja dizer:

1. One thing is to speak English, another thing is to understand (everything)
//
One thing is the speaking, another thing (completely different) is the reading (a habilidade de ler) / listening (a habilidade de ouvir)

2. One thing is to speak a foreign language, another thing is to speak your (native) language

Tenho dúvidas se o "another" necessita do thing de novo ou se é correto usar do elipse (ocultar) para tal palavra?

Espero ter ajudado!

To #1 could be the one or more of the following:

One thing is to speak English and quite another is to understand what they/the other person is speaking.

One thing is to speak English and quite another is to make up all the words they are saying (in the language, we assume).

To get by in English is one thing, to understand everything you listen to in English is quite another.

One thing is to know English, and another thing altogether is to understand all nuances and meanings of what is being said.

One thing is to be able to speak in English, another thing is to understand what is being said by others.

And other ways, notice that the meaning of "entender" (from Portuguese) varies with context.
You may understand - having to do with cognition; and you may "understand" meaning with "listening and identifying/making up" the spoken words.

Ah! And the "is one thing,...is another thing" with thing "at both sides" of the expression is okay, natural and usual. Used to compare two items and let others know that the second is more important/serious than the first. Or that the second is beyond comparation, sort of.

PPAULO escreveu: ↑10 Jan 2020, 07:14 One thing is to speak English and quite another is to understand what they/the other person is speaking.

One thing is to speak English and quite another is to make up all the words they are saying (in the language, we assume).

One thing is to know English, and another thing altogether is to understand all nuances and meanings of what is being said.

One thing is to be able to speak in English, another thing is to understand what is being said by others.

In these, it sounds more more natural to me to start with "It's one thing", or with the "ing" formulation of the verb, or the abstract concept behind the verb (e.g. familiarity instead of knowing):

"Speaking English is one thing, but it's quite another to truly understand what someone else is saying."

"Familiarity with English is one thing, and quite another thing altogether is to understand all the nuances and connotations of what is being said."

"It's one thing to be able to speak in English, and another thing to truly understand what is being said by others."

https://www.macmillandictionary.com/us/ ... t-thing-to

But after a quick google search, I guess some people do just start with "One thing is", but it just seems weird to my ears. Maybe it's more common overseas.

Thank you Bryan for your insight, I think you are right on that one.
It´s always good to have such insights and tips from people that use English as their mother tongue (or got up-close and personal with it). English never ceases to surprise the learner!