A 19th century language educator, M. Gouin, was obsessed with the language of German. His effort to learn German was herculean, people said: He knew everybody’s ‘Method,’ and learned the whole dictionary through and through, but shockingly, he found at the end that he did not know one word of German ‘as his nephew spoke.’ … So, after a ten months’ of his study at German, he returned to France and found that his little nephew—whom, when he left, was about two and a half years old, and not yet able to talk—had done what his uncle had failed to do just in such short of time.
“What!” M. Gouin said, “this child and I have been working for the same time, each at a language. He, playing round his mother, running after flowers, butterflies and birds, without learning, without apparent effort, without even being conscious of his work, is able to say all he thinks, express all he sees, understand all he hears … But I, knew all the grammar, versed in the sciences, versed in philosophy, armed with a powerful will, gifted with a powerful memorizing thousands of vocabularies . . . . have arrived at nothing, or at practically nothing!'”
(Home Education, 305, Charlotte Mason).
Language acquisition is basically what M. Gouin’s nephew and all children in the world naturally do everyday: playing and being fully immersed in day-to-day encounters with others and things in the world. There, they subconsciously (and wealthily!) acquire the language use embedded in the practice.
When it happens: when they are acquiring the language, they are usually not aware they are acquiring it, because it’s subconscious.
On the other hand, language learning is similar with what M. Gouin did two centuries ago and what many students still do in today. They heavily focus to the study of out-of-context grammatical rules and vocabulary memorizations for constructing out-of-context sentences.
While acquisition is a subconscious and intuitive process, study is very a conscious and cognitive about the mechanism of the language. So when we are studying about a language, we are aware that we are learning about a language: about its grammatical rules, its vocabulary, etc.