> > I always find it interesting that many people who learn English as a second language often understand and know the formal rules of the language better than those of us who've spoken it natively.

> I believe that's true for any natural language. To a native speaker, its rules are so natural (almost as breathing) that they hardly ever think about it. Frequently they don't even realize there's a question of picking the right form, because their brain does that job for them.

I personally rather think that most other languages have a much more complicated grammar than English. So many people learning English as second language are much more used to think of languages in terms of grammar. I, for example, also think of German (my native language) a lot in terms of grammar - OK, the reason might also be that I'm a really mathematical-minded person and heard some lectures about natural language processing (NLP), which further trained me in this kind of thinking (but I also remember that I already did so at the end of primary school).

I can say that some of the more obscure grammar rules of German are also not completely natural to many German speakers. But since in Germany at least in more educated circles there is a culture that using wrong grammar (as a native speaker) leaves a bad impression, you better know some rules to be able to explain why something was right or wrong. To give a common class examples, where a wrong dative instead of a correct genitive is used:

"dem Nachbar sein Sohn" (wrong, since for ownership you have to use a genitive and not a dative; roughly translated with "the neighbour his son"). Correct is "des Nachbars Sohn" ("the neighbour's son"; but this form sounds rather educated in German in opposite to English)

If you want to sound less stilted than "des Nachbars Sohn", you use "der Sohn des Nachbars" (Remark: if you want to troll Germans, ask them whether "der Sohn des Nachbars" or "der Sohn des Nachbarn" is correct. Answer is: both are, cf. http://www.duden.de/sprachwissen/sprachratgeber/beugung-von-...). This uses the correct genitive. Less educated people again use a wrong dative combined with "von": "der Sohn von dem Nachbarn", which is somewhere between very colloquial and wrong, because for ownership you use genitive
(both examples would be translated with "the son of the neighbour" in English, but the distinction between these two constructions can't be expressed in English).

Another problem is that there exist some verbs that have a genitive object (these often sound very educated), among these is "gedenken" ("to commemorate"). Because these verbs are not used so often, many less educated people use them wrongly with a dative (which appears more natural, but is wrong). For example "ich gedenke den Verstorbenen" is wrong; correct is "ich gedenke der Verstorbenen" ("I commemorate the deceased", where this grammatical subtlety again cannot be expressed in English).

TLDR: I rather see two reasons: 1. Other languages have a more complicated grammar than English, so that native speakers of other languages think more in terms of grammar. 2. At least in Germany in educated circles there is a culture that using wrong grammar leaves a bad impression. So you better know the rules why something is right or wrong.