CAUTION: NOT A TEACHER
Suthipong,
Congratulations! You are an excellent student, for you have asked a very
thoughtful question that has been asked by many people, native speakers included.
(1) Here in the United States, there are millions (yes, millions) of people who do not
use the third-person "s." (Of course, when I hear them speak, I go crazy. It sounds so
horrible to my ears.)
(a) Do you listen to American pop music? Well, some of those singers are people
who do not use the third-person "s."
(b) The government has told those people: It's fine if you speak that way in your
private lives, but -- please ! -- speak standard English at your jobs.
(2) Why do those people not use the "s"? Well, here may be one reason:
(a) In the 15th century, people who lived in the middle part of England stopped using
any ending for the third person.
(i) "John Dam kno" instead of "knows."
(ii) Shakespeare had one of his characters say: "the town is beseech'd, and the trumpet call (instead of calls) us to the breach."
(3) Some of those people came to the American colonies (as you know England used
to rule part of what we now call the United States of America) and spoke that way.
Well, over the years, educated people continued to insist on the use of the "s," but
some people, including those who did not attend school, continued to drop the -s and
this practice has continued (in some families) right down to 2012.
(4) Believe it or not, some people think that maybe in the future, English speakers will
decide to simplify the language even more and drop the third-person "s."
(5) BUT we are living in 2012. Please, please, use the third-person "s." If you do not,
people will think that you are ignorant and lose respect for what you have to say.
*****
P.S. I got all this historical information from the first volume (Syntax) of A Grammar of
the English Language (1931) written by (IMHO, of course!) the greatest grammarian
who has ever lived (and who will ever live): Professor George Oliver Curme.