If you are interested in the subject a very popular set of hypotheses at the moment come from Stephen Krashen of the university of California. He has helpfully put his book online: http://www.sdkrashen.com/content/books/principles_and_practi...

I will say that, like many studies in psychology, I find his research to be less than rigorously tested. I think it is reasonable to keep a skeptical eye, but even if the work is guided by intuition, it appears to be very good intuition.

Very quickly. There are 5 main hypotheses:

- Acquisition-Learning distinction hypothesis that states that language acquisition (ability to use the language freely without undue conscious thought) is different than learning (being able to remember facts about the language). Acquisition is required for fluency.

- Natural Order hypothesis states that grammar constructs in language are acquired in a certain order, no matter what order you teach the material. This has actually been show in studies and verified independently. Of the 5 hypotheses, I think this is the most rigorously tested. Somebody actually proposed the natural order for English grammar, but I can't find the link. It is (surprisingly) not very important what it is (see below)

- Monitor hypothesis states that there is a process in your brain which essentially scans language input and output and states whether it parses or not. For example, "Harold and me are going to the store", sounds wrong even if you don't know that "me" should be used as an object of the sentence, but is instead being used as a subject.

- Input hypothesis (the most important, but also most controversial hypothesis). Acquisition results largely/only from being exposed to input that you understand (input being language that you hear or read). In other words, in order to acquire language, it is sufficient to expose yourself to language that you can understand (in any way -- it being explained, obvious from context, etc, etc). Practice with output (speaking, writing) is not necessary (though, personally I think it may be helpful in some circumstances -- certainly helpful just from the acquisition of the physical skill of forming the sounds).

- Affective filter hypothesis states that language acquisition is hampered (and perhaps halted altogether) in situations of stress. Language acquisition occurs when the person is in a relaxed environment, without any pressure to achieve anything in particular (obviously a big problem for classrooms if true).

There is one other corollary which is quite important, known as "i + 1". It states that when language acquisition occurs, if your current ability is "i", then you will automatically acquire "i + 1" along the lines of the natural order hypothesis. As long as you are exposed to the material in a way that allows you to understands what it means (again, though context, explanation, etc) you will naturally acquire the next bit that you need. There is no need to teach to the natural order (and in fact Krashen suggests that it is counter productive because you limit what the students are exposed to).

So the upshot is that you simply have to expose yourself to language that you can understand and you will acquire language. There is no need (and it may be counter productive) to limit the types of grammar that you are exposed to because you will automatically acquire the next bit that you need in the natural order as long as you are exposed to it. Limiting the types of grammar you are exposed to will only slow you down. Learning (as opposed to acquisition) is not necessary, but can be helpful in strengthening your monitor (which is good for providing feedback). However, Krashen warns that too much reliance on the monitor (as is common in classroom teaching) will not lead to acquisition.

I would add a few personal observations. My experience is that these hypotheses largely fit what happened in my classroom and in my own study of Japanese. Krashen does not talk extensively about the role of memory in acquisition, nor does he talk about the need for repeating exposure to certain input. I think he is probably right that it is unnecessary to consider this, but my experience is that memory helps to speed the process by allowing you to understand more input.

This is why I suggest memorizing the meanings of sentences. If you do so, you will have more context for trying to understand new input. It's not really important for acquisition, per se, but it gives you a super fast look up table of examples so that you can understand more new input. Especially when coupled with reading (where you can progress at any speed that you like), this has dramatically improved the pace with which I could learn Japanese. It really appeared to help my students as well.

There are probably lots of study techniques that would work according to these hypotheses. Even the "study lots of grammar" technique is not at all bad if it is coupled with "read a lot of books and struggle through comprehending the sentences using the grammar rules you learned". The problem is that most teachers couple grammar memorization with language output, leading to students who know lots about a language, but if they acquire anything it is a kind of baby language of non-idiomatic phrases that they invent themselves (like... programmers... ahem...)

One other very important thing to realize is that there is not anywhere enough time to get the repetition necessary to acquire large proficiency in a language in the classroom. It must be self study!!! Again, this is where most people (and especially teachers) get it wrong. Even when I ask teachers, "How did you become fluent in X?", they never reply "at school". And yet, they still believe they can teach people to become fluent in a language at school.

Just as a caveat, there are some classroom techniques evolving which seriously limit the scope of language in order to teach fluency of a small subset in the classroom. This can work very, very well if you are simply teaching the students how to continue on in their own study once they gain confidence in their ability to become fluent in a small subsection of the language. Unfortunately, the hubris of teachers (like programmers) is largely unbounded and for some reason they think they can go beyond this role...

Hope you found this interesting. I really should put this content somewhere semi-permanent. I seem to type the same thing every few month ;-)