If language is as instinctive to humans as  dam-building is to beavers, if every 3-year-old is a grammatical genius, if the design of syntax is coded in our DNA and wired into our
brains, why, you might wonder, is the English language in such a mess?  Why does the average American sound like a gibbering fool every time he opens his mouth or puts pen to paper? 

The contradiction begins in the fact that the words “rule” and “grammar” have very different meanings to a scientist and to a layperson. The rules people learn (or more
likely, fail to learn) in school are called prescriptive rules, prescribing how one
“ought” to talk.   

Scientists studying language propose descriptive rules, describing
how people do talk — the way to determine whether a construction is “grammatical” is to find people who speak the language and ask them.

Prescriptive and descriptive grammar are completely different things, and there is a good  reason that scientists focus on the descriptive rules. —Steven Pinker, The New Republic