Acquisition vs learning is a dichotomy hypothesized by Krashen. It makes sense to me but it is impossible to convey to a skeptic the difference in feeling between coming up with or recognizing something you learned and something that just pops out. It actually feels different, but science can’t deal with that. Some tricky fellow will perhaps find a way to do so, but for now, those skeptics will just shrug their shoulders and repeat that they have no way to distinguish comprehension or production that is acquired from that which is learned.
Those of us who have seen the evidence in our own learning and in the classroom that students will show competence in L2 without direct instruction, based merely on having understood L2 which contained a particular feature, continue to teach as if it is an accurate description of what happens, i.e. that something we label acquisition occurs and is different from learning. (the question of whether or not something learned can transfer into the acquired realm remains open for many; Krashen has stated that each feature must go through the acquisition process, regardless of how well it is used when the learner’s monitor is “on”). As we do so, discussion ensues about how all the subtleties of a major feature like progressive tense or subjunctive mood or instrumental case does not need to be taught for students to acquire them; the acquisition will draw on the input, another reason not to shelter input.
(this latter point of sheltering input needs tweeking: tprs says it shelters vocabulary, not grammar, by which they mean they go for the meaning of words and put the words together however the language demands. In that process, the grammar is acquired or “picked up” without direct instruction. Questions about structure are given brief responses just to clarify or satisfy. The reference among tprs-ers to structures deals with phrases like “going to do X”, not “the use of â€˜go’ as a modal verb with the infinitive of a lexical verb”. My guess is that most tprs-ers do in fact shelter grammar but I don’t think that is the original intent)
The point of this entry is to point out that some subtleties of a feature cannot be threaded out and taught either explicitly or embedded in input b/c the teacher does not know them. Examples would be English perfect tense used to refer to past events whose impact is still felt at the moment of speaking or the present progressive used to refer to future events that are planned. Perhaps I am wrong, I haven’t talked to a lot of ESL teachers but I have talked to a lot of English teachers and they don’t know grammar like that. All they can tell a foreigner who writes using those features inappropriately is that “we don’t say it that way.” So, given a native speaker or a teacher who has achieved that level (very rare), it is entirely likely that the learner would acquire the subtleties, so that, for instance, a learner of English would not be found saying, “*When it rains, the picnic is being cancelled” but would say, “When it rains, the picnic is cancelled” and “It’s raining, so the picnic is being cancelled” (referring to the present and not planned) and “If it rains tomorrow, I’ll be cancelling the picnic” (planned).
All of that is very subtle and violations of those rules can give someone a foreign sound just as it does to Desi from the subcontinent who say things like, “Whenever it snows, I am cancelling my appointments” instead of “Whenever it snows, I cancel my appointments.” I didn’t asterisk the first one because such usage is pretty standard in Indian-English.