I recently had occasion to complete a (modern) language placement test. The test was composed of 20 sections, each asking me to manipulate given sentences and change the forms to other ones (e.g. the equivalents of present to past, one structure to another, and so on). It wasn’t an overly taxing test, though it did take a bit of time, but in terms of assessing my language ability, I would rate the test itself a fail.
Firstly, because I didn’t need to understand the messages in the text in order to manipulate them. Indeed, there was vocab in there that I didn’t recognise. But whether I could or couldn’t understand the texts, was irrelevant to the task, which was transform structure to structure. That only required an explicit grasp of particular grammatical forms.
Secondly, there’s no guarantee I can reproduce this level of grammatical correctness in speech. In fact, I know that when I speak I am producing all sorts of errors in these forms. I would like to ‘fix’ that, but I doubt that explicit instruction in grammar will help that because I explicitly know all this grammar. “You should know this by now”; “we’ve been over this grammar X times, why are you still saying it wrong”; “You can explain this grammar, why can’t you produce it in speech?” – these are all things (some) teachers say, and they are all predicated on wrong beliefs.
I like explicit grammar. I love learning about linguistics. I think there’s a small, but non-zero place for it in language education contexts. But I’m pretty convinced that manipulation of forms does not lead to acquisition – not to communicative ability but also not even to real-time correct use of forms.