Incorrect grammar usually results from lack of knowledge; the individual
does not know the correct, so continues the "habit" of using the incorrect.
 Sometimes an individual will deliberately use incorrect grammar in order
to "fit in" with a group that does not use correct grammar.  Back in the
early 70's there was a judge from Arkansas who had practiced law in
 Washington D.C. before deciding to run for Congress.  When he returned to
Arkansas to campaign for election, he removed a dental plate with a false
tooth in the front of his mouth because he believed it made him "one of the
His sophisticated manner of speaking in Washington changed as well.

For decades, as I introduced myself as a "speech teacher," I was frequently
surprised and somewhat amused by the number of people who would respond, "I
need to take your course," or "I need to speak more properly."  Usually,
"educated people" who associate with other "educated people" tend to lose
some of their local or regional misuse of language.  The percentage of
people who like the way they sound and/or are satisfied with the way they
speak is very small.

Acceptability of language changes evolves downward, unfortunately.  The
British tend to be shocked at what Americans have done to the English

I believe that Americans' reliance on television for information and
entertainment rather than reading novels has diminished our knowledge of
correct grammar.  The typical television sit-com is written for an 11-12
year old intellect.  Add to the mix the fact that regional dialects (The
Nanny, for instance) and "red-neck" humor abound in programs and
commercials, and you have the "dumbing down" of the English language.  I
find it interesting that the Singing Cowboys (Roy Rogers & Gene Autry)
never sounded like ignorant hillbillies, although their "comic relief"
sidekicks did.  Gunsmoke star James Arness was a Canadian whose
language/speech never called attention to itself.  Festis Hagan's speech
brought a smile to us because it was out of the ordinary.  There was a
reason why Festis would not likely become the sheriff.

Professional people do not want the way they speak to call attention to
them.  Rather, we want listeners to focus on the message.  John F. Kennedy
had a speech coach after he was elected because he wanted to reduce the
regional identification of his speech.  The way he grew up speaking in
Massachusetts caused him to pronounce Cuba as Cuber and Miami as Miamer.
 Wrong in his region? No.  Did it call attention to itself? Yes.  Some of
his later speech recordings contain the words being pronounced both ways.
 Apparently, as Kennedy became excited or passionate about his subject, he
tended to revert to his regional habits.  After Kennedy was killed an
Johnson was sworn in, a friend of mine in New York heard a colleague say,
"Oh, no, not Johnson.  He speaks with an accent!"

I had a colleague in the Dallas area back in the 70's who was a speech
pathologist (I believe the field is now referred to as communication
disorders), who began to get clients in the Dallas area who wanted to
reduce their regional speech identities.  Although most were actors,
business people, and television/radio personalities, one was an Iranian

The only way to know correct grammar is to study and practice it.

I probably answered in more detail than you asked, but I enjoyed the
exercise.  Hope it helps.