I´m in doubt about it. When you´ ve achieved a certain level of knowledge of a language, it even gets harder to advance. After having internalized the basic grammar and vocabulary efficency of language learning seems to decrease, because you have to spend the same time as before to get less progress. It´s the fine tuning which may cost years. You have to learn more detailed stuff which doesn´t concern the language in its entity anymore, but is appendant to a small group of words and situations only. That´s why you often forget these hard acquired facts as well: There aren´t so many opportunities to repeat them in conversations. Even if you live in an ambience of native speakers, it´s like that: The more you know, the harder it gets to convince them that it is still necessary to correct you -- even if your speech is understandable. Well, that´s why I´m here actually. ;)) I hope that people will correct me, even if I only use an unusual expression.