"This book is not understandable."
Translation:Tiu ĉi libro ne estas komprenebla.
I have got two problems with this change.
The first one is philosphical: Mal- is the opposite of something. This is only the same as as the negative if it binary. You can be pregnant or not, but nothing in between. But between good an evil is wide range. Therefore “not good” does not necessarily mean “evil.” So is between understandable and incomprehensible.
The second one is linguistical: In Englisch the negation is often used to emphasize the opposite. “The Queen was not amused.” means that she was upset. “That’s not bad.” means that it is very good.
To me "That's not bad" in English means it's reasonably good, but not as good as I was expecting and could be improved upon. I would never take it to mean "very good" at all.
It's just as well that in Esperanto we have so many prefixes and suffixes that we can use to add subtleties of meaning and try to overcome cultural miscomprehensions, cxu ne?
But in this case, I agree that "ne estas komprenebla" more closely matches what the English is implying. I was really just thinking aloud :)