My Grammar and I (Or Should That Be 'Me'?): Old-School Ways to Sharpen Your English

The Oxford comma
The Oxford, Harvard or serial comma is placed before the final and,
or, or nor in a list of more than two elements. (The names are derived
from the Oxford University Press and Harvard University Press, both
advocates of this usage.) Most (British) pundits recommend it only to
avoid ambiguity.
He  introduced  me  to  Mr  Brown,  his  teacher  and  his  friend.  (He
introduced me to one person, Mr Brown, who happened to be both his
teacher and his friend.)
He  introduced  me  to  Mr  Brown,  his  teacher,  and  his  friend.  (He
introduced  me  to  two  people:  his  unnamed  friend  and  a  teacher,
whose name was Mr Brown. Or indeed, if the speaker is particularly
grammatically unaware, to three people: Mr Brown, plus the teacher
of  either  Mr  Brown  or  the  speaker,  plus  a  friend  of  the  teacher,  Mr
Brown or the speaker.)
Oxford  commas  are  particularly  useful  if  one  of  the  items  in  a  list
already  contains  the  word  and  ,  or  in  sentences  such  as  this  one,
where the items of the list are a complicated collection of phrases or
clauses.  Dick  King-Smith’s  novel  Poppet  contains  this  perfect
example: ‘He asked beetles and grubs and worms and caterpillars and
little  lizards  and  small  frogs,  and  some  replied  jokily  and  some

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replied angrily and some didn’t answer. ’
For  items  already  containing  the  word  and  ,  Wikipedia  quotes  The
Economist  style  manual:  ‘The  doctor  suggested  an  aspirin,  half  a
grapefruit and a cup of broth. But he ordered scrambled eggs, whisky
and  soda,  and  a  selection  from  the  trolley.’  That  comma  after  soda
tells  us  to  treat  whisky  and  soda  as  a  single  item.  Though  from  the
sound of this guy, it was probably a double.
  8.   In large numbers. In numbers of more than three digits, use a comma after
every third digit (reading from right to left):
I make that 6,000 people.
20,000 leagues under the sea.
The population of Argentina is 34,663,000.
But  note  that  in  scientific  texts  and  particularly  in  tables  of  figures  the
comma is often replaced by a space: 6 000 , 20 000 , 34 663 000 .
   9.     Around  a  non-restrictive  clause.  Be  careful  with  commas  here.  They
change what you mean to say:
I pulled up
all the
flowers
which
looked like
weeds .
Restrictive: I pulled up only the flowers that
looked like weeds.
I pulled up
all the
flowers,
which
looked like
weeds .