Alissa G. answered 03/26/19
M.A. Spanish; Language Expert with Experience Living Abroad
Hi there! Yes, generally speaking, in Spanish we see perfectly acceptable double negatives where we would not in English. I would normally translate Hasta que as until, rather than if (if would be si without an accent over the letter I). However, Quijote is written in a much older Spanish. So, if the book is saying that hasta que actually means "if" here, then yes, the sentence reads correctly: According to the laws of chivalry, he could not fight anyone if he was not knighted (meaning he had to be knighted in order to be able to fight).
You could also think of it this way: According to the laws of chivalry, he could not fight anyone as long as he was not knighted. The negative structure is acceptable here. The sentence still conveys a clear message.
I hope that helps! Feel free to message me if you ever want to have an online session.
Warmly,
Alissa G., M.A. Spanish