When do you really know a piece of a language?
Usually, we say we "know" something if we can remember it and understand it. When you're learning a language, it is easy to think you "know" a piece of the language, when really, you are still only halfway there.
The problem is the difference between two kinds of knowing, which are called "knowing that", and "knowing how". "Knowing that" is the kind of knowledge we have about facts. For example, you probably know that the capital of Japan is Tokyo. In order to know this, you only need to be able to remember it when you need to, and understand it.
But, "knowing how" is the kind of knowledge you have when you swim, or drive a car. Here, knowledge of the idea is not enough. For example, I might have read several books about how to fly helicopters, and I might be able to remember the ideas. I might understand and explain all the things you have to do to fly a helicopter. Unless I have practised really flying a helicopter and shown I can really do it, you will not want to climb into the helicopter with me and let me fly you across the city. Pilots know how to fly a helicopter.
When we are learning language there are all kinds of "knowing that" which can be useful to us (grammar etc). But ultimately, language is really a matter of "knowing how". We have to learn not just the ideas, but how to actually do it, in real time (= fast!). This means that even when you "know that" something about the language is true, you have not really "learnt" it. You still have more work to do.
There are three steps to getting to "knowing how". Only the first one is "knowing that":
These three steps, put together, mean that learning any one thing really well takes time. You should be patient, and you should be prepared to have several different things (words, rules) that you are trying to learn at the same time.