Una cosa ... otra cosa

Here's a useful construction that I haven't seen mentioned in Spanish textbooks. It is used to identify an analogy as false. It equates with the English expression it's one thing to ... although the grammar in Spanish requires special attention, as we'll see.

• Una cosa es escribir cuentitos de secundaria o de prepa y otra muy distinta es hacer literatura. It's one thing to write up stories for school. But to create literature — well, that's something entirely different. (movie dialogue)

As you can see, the formula here begins with una cosa es. The second part will be some variation of otra ... es, for example, otra muy distinta es or otra cosa es.

• Una cosa es contarle tus broncas a un doctor así y otra cosa es contarlas delante de todo el mundo There's a big difference between opening up to a psychologist and blabbing your secrets to the whole world. (telenovela dialogue)

In the examples so far, es is followed by an infinitive. The following examples demonstrate that when the complement of es is a clause introduced with que, we need subjunctive. The speaker is not asserting the proposition expressed by the que clause here. Instead, they are conceding it for the purpose of making an argument.

• Una cosa es que no estemos en el mismo grupo. Otra cosa muy distina es que no hayamos olvidado de ti. It's true that we're not in the same group anymore, but that doesn't mean that we've forgotten about you.
• Una cosa es que nos parezcamos y otra muy distanta es que seamos iguales. Just because we look alike doesn't mean we are the same person. (twins)

Keep an eye out for opportunities to use this construction in your conversations. It is a possible response whenever an apples-to-oranges comparison is being made.