Translation:Esa carta es acerca de ella.
That's what I thought. I certainly don't see how it's wrong. If it is, perhaps someone could clarify?
You are right, can only use su when you are talking about something that belongs to her
You would not use sobre. That means about in the literal sense -- about, on or around.
What's wrong with the translation of letter as letra instead of carta in this sentence?
Letra is a letter if the alphabet, it is only the same word in english, in spanish it is not the same thing
Why can't I use "Aquella" since English doesn't distinguish a near and far "that"?