a/an and one

A     a/an and one   (adjective)

1      When counting or measuring time, distance, weight etc. we can use either a/an or one for the singular:

           £ 1      =   a/one pound                               £1,000,000      =       a/one million pounds

But note that in the rent is  £100  a  weak the a before week is not replaceable by one.

In other types of statement, a/an and one are not normally   interchangeable, because of one +  noun normally means ‘one only/not more than one’  and a/an does not mean this:

                               A shotgun is no good.  (It is the wrong sort of thing.)

                              One shotgun is no good. (I need two or three.)

2      Special uses of one

      (a)   one  (adjective/pronoun)  used with another/others:

              One (boy) wanted to read, another/others wanted to  watch TV.

              One day  he wanted his lunch early, another day he wanted it late.

      (b)   one can be used before day/week/month/year/summer/winter etc. or before the name of the day or month to denote a particular time when something happened:

                 One night there was a terrible storm.

                 One winter the snow fell early.

                 One  day a telegram arrived.

           (c)    one day can also be used to mean ‘at some future date’:

                    One day  you’ll be sorry you treated him so badly.

             (Some day would also be possible.)

B       a/an and one (pronoun)

one is the pronoun equivalent of a/an:

                 Did you get a ticket?  — Yes,  I managed to get one.

The plural of one used in this way is some:

                 Did you get tickets? —  Yes, I managed to get i some