An Englishman who has been learning Spanish on and off for 4-5 years, and has been living in Spain for 13, came to me to complain about a peculiar way of expression used a lot in Spanish, but makes no logical sense at all to a foreigner, English or not (I guess, because I don't know that many languages). It's what we would call 'double negative'. He even brought the newspaper along to illustrate his point, pointing at the news about the King being operated on one leg last night. It says 'The King will have to postpone all his official engagement planned for a while, 'until he doesn't recover fully'. The English way of saying the same thing would have been 'until he fully recovers.'
Other example: I will wait as long as necessary until you 'don't come' (instead of until you come or show up.) Greek or double Dutch to many.
This is one of the most confusing use of grammar in Spanish for foreigners trying to pick up some Spanish, I agree; but the very first thing to learn a new language, any language, is to forget totally one's own language, and never try to compare or translate any sentences word for word. Or you would never, never get it right, ending up speaking 'pigeon Spanish'.
Some of the English I know around here declare they speak fluent Spanish; the fluent part is right, but that doesn't mean they speak Spanish correctly, which often confuses the Spaniards, making it hard for them to understand. That, in turn, confuses the English, because they do use all the correct words but these are not put in the right order or combination. Simply because they reckon it doesn't make (English) sense, forgetting that they are trying to speak Spanish.
There are many, too many perhaps, peculiarities in all languages. I have endless trouble explaining to the Spaniards what exactly 'How do you do?' mean. They all want to know 'Do WHAT?' and WHY such a question when you are not supposed to get an answer!
