A/an and one both refer to one thing and are used with singular countable nouns.
We usually use one if we want to emphasise the number.
Compare: It took just an hour to complete.
It took just one hour to complete.
We also use one with day when we are thinking of one particular day but we don’t say exactly which.
eg. I’ll go to the doctor one day next week.
We use one in phrases with one… other/another/the next
eg. He moves from one job to the next with no plan for the furure.
NB: We use ‘a’ with singular countable nouns in exclamations.
eg. What a lovely day!
Few, A Few, Quite, Quite a Few
Quite a few = a considerable number
eg. Quite a few people turned up at the free concert last night.
A few = a small number
eg. Peter and Jane only invited a few close friends to their wedding.
Few = not many or not enough
She has few friends. (formal) = She doesn’t have many friends. (informal)
NB: little/a little is used with uncountable nouns in the same way as few/a few is used with countable nouns.
We can make comparatives with fewer with countable nouns. We use less with uncountable nouns.