My goal in language learning is fluency. I want to read the newspaper and books. I want to talk to people, not as an idiot, but as an educated person. The most frequent 1000 or so words typical y account for 70-80% of most content. But most of the key meaning is often in the missing 20-30%. Since these less frequent words appear so infrequently, they are difficult to learn, unlike the high frequency words that appear often and are easily learned.
With a strategy of concentrated reading and listening to authentic content, coupled with an intelligent vocabulary learning system, any learner can reach educated fluency.
In golf most teachers emphasize using the large muscles of the back, hips and thighs, rather than the small muscles in the wrists and hands. The large muscles are more stable, easier to control and give stability to the golf swing. The wrists and hands can come later. In many cases they take care of themselves if the big muscles work properly.
Language is similar. To me the big muscles of language learning are the important words and phrases - the key words that describe people, things, and actions. The small muscles are the details of articles, prepositions, verb or noun endings etc. Unless you have the big pieces in place, the little pieces do not matter. You can communicate well with a big vocabulary of words and phrases without knowing much about the smaller details.
Many textbooks, however, focus on the small details. My Russian text starts right in with talking about the genitive and dative case and when they are used. The book introduces different tenses and other rules. I just ignore them. If I can learn the words for the key people, things, and actions, that is all I need for now. If I learn the nouns and verbs as parts of phrases that I start to recognize, the little pieces will slowly fall into place. If I read and listen a lot my language will become more and more natural.
But I need to continue listening, reading, observing the language and reviewing words and phrases. I must not just think I have reached my level of competence in the language and then stagnate in the language as so many people do. If I keep listening and reading and learning I can continue to ignore the rules, drill s and questions and still improve, all the while enjoying my studies, without any pressure to be perfect.
Correct usage does matter, but I doubt that a lot of explanations will help achieve correct usage. Alice Walker says in her email "who real y cared between the difference of 'du' and 'de'
and 'de la' ―Surely the question is not whether it matters whether you say "de," "du," or "de la" in French. It definitely matters.
The question is how to most easily learn to get it right. Explaining the principles wil not help a lot if it is not possible to remember the gender of nouns. It is only with massive input via listening and reading that the learner's natural accuracy in the use of gender improves. It also helps to have mistakes corrected as long as it is understood that mistakes are normal and the corrections are used to help the learner become more observant of the language in her listening and reading.
I once bought a book of Beijing slang when I was studying Chinese. I have occasional y bought books with slang expressions or even other specialized dictionaries. They al gather dust on my shelves until I throw them out.
Why am I not interested in these? If I think about it, there are several reasons.
1) I am not that interested in slang. I just instinctively know that it is not something that I am going to use very much. When I am confronted with unknown slang expressions in movies or in real life, I just let them go by me. They are just part of the many aspects of a new language that I do not understand for now. If certain slang expressions appear often enough, I will pick them up, at first, well enough to understand them, and with continued exposure, well enough to use them.
2) A dictionary of a specifically defined range of terms, whether slang, or technical terms, or political terms, or newspaper terms, or whatever, is not a learning tool for me. I have never been able to read these dictionaries and then go out and use these terms. I have no sense of the context if I just read something in a dictionary. Nor can I remember terms that I deliberately study in a dictionary.
3) A dictionary is, at best, a place to look up words that you encounter in other contexts.
The more complete the dictionary the better. It is unlikely that in reading a text on politics, it is just the political terms that wil give me trouble. So if I am going to use a dictionary it is better to use a general dictionary, a very complete one.