Most students of Japanese find learning the language frustrating because it takes them so long to develop enough proficiency to communicate anything more than the simplest of concepts. Perhaps some of this is due to the vast linguistic difference between English and Japanese, but it is probably mostly due to current educational theories of teaching foreign languages that claim that languages should be learned through a process of gradual absorption, mimicking the path that infants follow towards competency in their mother-tongue. This is certainly a great step forward in the improvement of teaching foreign languages, a major improvement from the days of educators relying solely on translation and memorization of literature. In many places languages are still taught in this manner, particularly in Japan, where most Japanese people learn to read and write English by translation, producing people who can muddle their way through a research paper but can't hold up their end of an ordinary conversation. But in my opinion this process of learning by absorption is only useful to a point, after which it becomes more of a frustration and a hindrance to further study. Human beings are thinking creatures who have great skill at analysis and problem solving. Presenting language as something to be absorbed, like a form of aesthetics, is in my opinion only useful to prevent the beginning student from being overwhelmed by enormous amounts of complex rules and information.
At some point the beginning student of Japanese becomes an intermediate student of Japanese. They can discuss the weather, their hometown, and rattle off introductions without much problem. But when it comes to actually talking about something, producing new sentences with complex meanings which transmit new concepts from one person to another, there they begin to be frustrated by lack of knowledge or proficiency. Part of this is a problem of vocabulary. The student just doesn't have the words at hand to say what they want. This is only solved by memorization, adding new words to the mental dictionary. But the other part is the incapability of producing complex sentences, statements with more structure than just "A is B" or "I do X". It is the realization that the student lacks such a capability that marks them as having passed from the beginner to the intermediate.
The intermediate student has a hopeless task ahead of them if they expect to learn all the varied verb and adjective conjugations, the particles, conjunctions, aspects, and moods of Japanese only through gradual immersion. Adults and teenagers lack the intense absorptive capacity of language learning that is inherent in children, a fact which has been proven a number of times by scientific research and analysis. Instead the intermediate student should apply another innate facility that adults have in far more advanced a form than children, the capacity for logical thought and problem solving.
This Dirty Guide to Japanese aims to provide the student with all the tools necessary to start solving problems in Japanese. It gives the student the most important parts and pieces of the whole language, letting them take them into the field to try them out, to attempt real and useful communication with Japanese speakers. The descriptions of Japanese here attempt to be as comprehensive as possible in a limited space, presenting as many of the major aspects of the language as can be compressed into a short document. But it does not aspire to completeness, since a complete grammar of Japanese would require an enormous amount of effort and thousands of pages of detailed analysis. Such things are better left to the professional linguist, and indeed there are a number of very good reference grammars of Japanese out there that attempt to fulfill this lofty goal, some more successful than others.
This guide has very little to say about nouns. Nouns in Japanese are almost entirely uninflected. It's only a simple matter of memorization to add new nouns to your mental dictionary. You can learn vocabulary from a dictionary or simply ask a native speaker. Or just point and say "あれ" or "あのもの" or something. But you can't do the same with a verb conjugation because conjugations are not usually found laying around for you to point your finger at. Therefore this guide will focus on verbs and adjectives and how to inflect them, and about particles which can be used to string nouns, verbs, and adjectives into coherent sentences.
Please remember however that this is merely a guide, not a complete map to the langauge. There are many exceptions to the rules presented herein, and there are many other subjects which aren't touched upon here. Keep this in mind, but if you try hard to say what you want with the methods presented here you should be able to at least make yourself understood, even if you aren't saying it correctly.
This guide appears to be pretty comprehensive. The astute reader may begin to wonder why their textbooks don't provide a similar summary, or why things are not taught to them in such a complete manner, but instead over a long process of gradual increase. Truth to tell, there are a number of reasons for this. Among them is the fact that a school has to keep to the median level of understanding among its students, so that the people in the bottom half of the class aren't left behind while the top few students race ahead. Also, a school has a financial stake in extending the process of learning over a long enough period of time so that it can make a profit or at least break even in the end. The same is not so for the individual learner. Indeed, most students of foreign languages have heard stories of or even know people who have gone to foreign countries and learned in months what the average student takes years to absorb. So the real answer to the question of why this guide can be so comprehensive is that it is aimed not at classroom use, but at the individual looking to increase their own proficiency without worrying about keeping pace with other students.
One last thing should be said. This guide originally stems from The Quick and Dirty Guide to Japanese written by Tad Perry in 1992, firstname.lastname@example.org. The present author has put an extensive amount of effort into reformatting, editing, and rewording the text. The original roomaji has been converted into UTF-8 encoded kanji and kana. Examples have been corrected or reworded for clarity, and a large number of changes have been made, extending the document's reach. So much work has been put into it that I have felt uncomfortable in keeping Tad Perry's name as the author, since many of the statements and opinions are now mine, not his. Thus I've renamed it to The Dirty Guide to Japanese. It's no longer quick because I linger much longer on certain aspects of grammar and usage that the original skimmed over. It's still dirty because it's not a piece of professional work, just a sloppy reference that can be read for its good parts and then used for more productive purposes like wrapping fish or lining the bottom of bird cages.