"If I had to reduce all of educational psychology to just one principle, I would say this: The most important factor influencing learning is what the learner already knows. Ascertain this and teach him accordingly."
This quote was taken from a book by David P. Ausubel, one of the most influential figures in educational psychology. It was used in his book Educational Psychology: a cognitive view, where it came directly after the dedication, and before the preface. It is the first thing the reader will see in the book, and this is by design, because this is the most important principle that every reader should have in mind while reading the book.
To many, this may seem trivially obvious, and something that every teacher does. After all, you couldn't teach someone to do ski jumping if they haven't learnt to ski downhill already. But there's actually more to it than that, and the subtlety isn't in the words used in the quote, it's also about the words that he left out: words such as "subject". This is crucial. If he had set "what the learner already knows about the subject", then it would include the possibility of a student in a state of zero knowledge, but as it stands it assumes that there is no-one who enters the classroom without a considerable amount of pre-learned knowledge.
The error made by many teachers is to attempt to compartmentalise knowledge, and the subsequent belief that such "zero states" exist, and this is particularly prevalent in the language classroom.
But we all have different levels of knowledge.
A Spanish person knows what definite and indefinite articles are, although his native concept doesn't quite match the usage in English. A Polish person has no concept of articles whatsoever. These are very easy indeed to ascertain, yet we still don't tend to teach them according to these. The profession has in general bought into the myth of a "Universal order of acquisition" and the myth that there's "no such thing as native language interference", and there are hundreds of language courses on the market that pay absolutely no heed to the learner's native language. In fact, Cambridge themselves produce a series of books on "common errors" from their exams, all completely in English and all sold worldwide with no attempt to tailor the teaching to the specific problems that speakers of certain languages have.
What's to blame here is the obsessive fear of "translation" in the industry. Avoiding translation has become associated with avoiding any sort of acknowledgement of the existence of the native language. We are expected to turn it off -- all that knowledge, and we're not supposed to use it.
But we do use it. As I've no doubt said here on many occassions, when we are taught "hello, my name is ... , how are you?" in any beginner's class, we are learning concepts that only exist in language, so we are going back to our native language. We do it in the classroom; we rely on it in the classroom. But we pretend we're not doing it, which prevents us developing more advanced strategies based on the principle of native language as "what we know". We limit ourselves through the lie. Once we, as an industry, accept the truth, we can start to improve.