as co-ordinating conjunction
A comma (,) generally indicates pauses in speech.
But, when it joins two clauses, it indicates a contrast
between two ideas. In speech it is normal to draw attention to this
contrast by a slight pause. A comma is the usual way of indicating
this, although it is not obligatory:
- Sheila can eat anything and large quantities of it, but she
never puts on weight.
- I'm going to make some New Year resolutions, but I don't
suppose I'll keep them
A comma after but would be very unusual, Yong. Although
we might pause after but if we are making up our minds about
the consequences of something, this would not normally be indicated
in writing, unless we want to indicate hesitation with a discourse
marker such as er, reflecting informal speech:
- You can play in the fields down by the river, but not in
- You can play in the fields down by the river, but, er, not
in the woods.
and as co-ordinating conjunction
Note that and when it is used as a coordinating conjunction
indicates addition. In speech the addition of ideas is not
normally marked by a pause, so no comma is needed, unless the items
being connected are very long. Compare the following:
- The general announced that there would be a ceasefire and
ordered all his troops to lay down their weapons.
- Jennifer decided to try the home-made steak-and-onion pie
with mashed potato and buttered carrots, and David ordered the
breaded fish with chips and peas.
defining relative clauses
Defining relative clauses identify the noun being referred to.
They tell us which person or thing is meant. They follow immediately
after the nouns that they modify with no pause in speech, so no
comma is used in writing:
- The boy (that) I dated when I was at school has now married
my best friend.
- The shares (that) he bought when he joined the company are
Note that if we leave the identifying relative clauses out, the
sentences do not have very much meaning:
- The boy has now married my best friend. (Which boy?)
- The shares are now worthless. (Which shares?)