many English don't understand grammar

http://www.sciencedaily.com/releases/2010/07/100706082156.htm

Dr Dabrowska and research student James Street then tested a range of adults, some of whom were postgraduate students, and others who had left school at the age of 16. All participants were asked to identify the meaning of a number of simple active and passive sentences, as well as sentences which contained the universal qualifier "every."

As the test progressed, the two groups performed very differently. A high proportion of those who had left school at 16 began to make mistakes. Some speakers were not able to perform any better than chance, scoring no better than if they had been guessing.

She adds: "Our results show that a proportion of people with low educational attainment make errors with understanding the passive, and it appears that this and other important areas of core grammar may not be fully mastered by some speakers, even by adulthood.

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D - Sounds like an argument to avoid multiple ways to structure the same statement.
If these results apply to the first childhood language, then imagine for a second adult language!