I always confuse the difference between a/an and one, Where we use a/ and and one?
Use 'one' where the number is important or unexpected.
Also an is used before a noun or adjective describing a noun, starting with a vowel ie an apple, an agreement, an edible fruit, an iron, an irrestable dress,
an orange, an umbrella, an open umbrella, A E I O U
Yes, but it's before a vowel sound, so we say 'an umbrella' but 'a university, 'an egg' and 'a European'. We can also take 'an MA'.