1.The word “one” is pronounced as “won” in English. So it starts with a vowel but has a consonant sound. Hence for words starting with “one”, we need to use the article, “a” before them.
a one-rupee coin
a one-day program
But, “only” is pronounced in the same way as it is written. Hence we use “an” before only.
an only child
2.Words starting with the letter “u”, will have the pronunciation as “you”. For those words, we use the article, “a” before them.
This should not be followed for all words starting with “u”, since many words don’t have the sound of “you”.
3.Words starting with “Eu” will also have a consonant sound “you”, though it starts with a vowel ,e. Hence we should use the article, “a” before them.
a European country
a eucalyptus plant