NanakiXIII said:
It's been said that de was desu. It's been said that de was a particle. I don't know who's right and who's not.
Nanaki, no offense, but I'm afraid you just seem to be confusing yourself further and further the more you try to think about this. Go up and read CorDarei's most recent post again. "De wa arimasen/ja arimasen" is the negative of the copula "desu" -- know what it is, and learn how to use it. This is all you should concern yourself with at this point.
These other questions you're trying to answer -- what exactly is this "de" that's part of the copula, where does the "wa" come from, and how does it all interact with "arimasu"? -- are interesting topics for those in Japanese linguistics to consider, but not something that someone at your level needs to be spending all this time agonizing over.
No teacher or textbook would ever try to explain this to a first-year student, because it would just create unnecessary confusion and mental stress, and waste time that could be better spent on learning things like how to actually _use_ the basic grammar that's being studied.
The way I see it, there are two types of questions you can ask: those for which the answers will actually help you understand and use the language better, and those which may make you curious, but in the end aren't really worth the time spent worrying about them. You have to learn to tell the difference, and to ask (of your teacher and of yourself) the questions of the first type as much as possible, while leaving those of the second type alone except as an occasional diversion. If you don't, you're going to be tearing your hair out long before you ever achieve real proficiency in Japanese.
I doubt this is the kind of answer you were hoping for, but I think it's good advice for the beginning student. What you're doing to yourself would be like someone obsessing over how the engine of a car works before taking their first driving lesson. It's natural to be curious, but there are some things that you don't need to understand to be able to drive a car.
TZed9 said:
To get round this, my plan is too learn very basic nihongo, then go and live there for at least a year.
Listen to natives has to be the most certain way of getting the concepts right! Wouldn't you agree?
This is a common misconception of the beginning student. That somehow you can go to Japan starting from zero (or near-zero), and come back respectably fluent in a year or so's time. It might possible if you were _very_ gifted in languages and a _very_ hard-working student, but in almost any situation, you will progress _much_ more quickly if you develop a strong foundation in the language (say, the kind that could be achieved by two or three years of formal study) _before_ you go to live in Japan.
There will come a point in your studies at which listening and talking to natives (or reading materials written by/for natives) will become by far the best thing that you can do for your Japanese, but at the early stages, the right kind of structured learning is _far_ more valuable. Otherwise, you're just going to find yourself spending the better part of your time reinventing the wheel (and reinventing it wrongly all too often).