Please tell me the difference between a million and one million. Many thanks!
If you want to stress the magnitude of something you say ‘a million’ as in ‘there is a million reasons why you should learn English’ while ‘one million’ is just an exact number.
Exactly as Torsten says and the same applies to -
a/one hundred, dozen, thousand, trillion.
And, while writing out the number in words, we invariably say one hundred/thousand/million as the case may be. That merely quantifies it.