One might master the grammar rules of a second language and still go misunderstood with an utterance. My gut feeling would say that is caused by the employment of unusual constituents. So the degree of how intelligible an utterance sounds could be schematically laid out as follows:
- Grammatical/Usual (natural) - e.g. "English is not my first language."
- Ungrammatical/Usual (solecism) - as in the double negative in "Ain't nobody got time for that."
- Grammatical/Unusual (weird) - as in a literally translated idiom. In (Brazilian) Portuguese, one can say "I had to swallow that toad" if someone else went off on that one and the latter had to take it -- for whatever reason -- without getting back. It's a perfectly grammatical sentence, comprised of very common words, yet no English speaker would understand that without an explanation like this (I think).
- Ungrammatical/Unusual (unintelligible) - e.g. "The arguably potatoes consider I."
(Bear with me, it would be better readable if a table was possible)
I wonder how valid that is from a linguistics perspective. I've been struggling to tame English and learning other languages as well and that schema occured to me as a potential instrument to real language acquisition.