If you have often heard, or said, that language learning is a bit like a spiral, Nigel Caplan takes this metaphor even further: it’s a tornado because it expands (and is probably just as chaotic, though hopefully not as destructive). You can read more about this in his post if you need any more convincing! For example, if you say, ‘We’ve already done the Present Perfect’, you can’t be more wrong, either as a learner or as a teacher. It’s never over because there are always new and more complex contexts and situations where this language could be applied, and the knowledge doesn’t transfer automatically.
So, next time a student comes to me with a question about his or her sense of progress, I know what picture I’m going to draw!