difference between a/an and one

a/an - Using one puts more emphasis on the number

one    - A/an and one both refer to one thing, sounds more natural

 

difference between a/an - one...

(hi, i would like to know the difference between the word use "a/an" and "one";in other words, when is used these words in a phrase?

for example:

"please, give me one cigarette" or "please, gimme a cigarette"

i just use each one instinctively but i don't know its difference...

if you could help me, i'll thankful to you)

• A/an and one both refer to one thing. Using one puts more emphasis on the number. Examples : Henry gave the taxi driver a note (not a coin), Henry gave the taxi driver one note (not two).
  
  We use one (not a/an) when we mean one of a larger number :
  One question/One of the questions in the exam was more difficult than the others
  The team wasn't at full strenght. One player/One of the players was injured.

• Hmmm...good question.  I think. In your example, "gimme a cigarette" sounds much more natural than "give me one cigarette". If you said "gimme one cigarette" the other person might say (jokingly) "just one? sure you don't want 2 or 3 or the whole packet!?"
  I haven't really answered your question, but in general, if you need EXACTLY one of an item, you are OK to say "can I have one <item X>, please".