In English grammar, it is not normal to use the word ‘a’, which can mean ‘one’, before a vowel (a,e,i,o and u, and the silent ‘h’), so before vowels, the article ‘a’ changes to ‘an’:

e.g. a car an elephant a house an ice-cream a boy an old car an hour

*Note that the changes in the spelling and pronunciation of the articles depend on the pronunciation of the words that follow the articles, but not on their spelling alone, because though some words begin with vowels, they do not give out vowel sound, for example, though the word “university” begins with ‘u’, a vowel, it is not pronounced with the same sound it has in the word ‘uncle’, therefore, when using an article before it, we use ‘a’ but not ‘an’. And some words beginning with consonants (-,b,c,d,-,f,g, (h),-,j,k,l,m,n,-,p,q,r,s,t,-,v,w,x,y,z) give out vowel sounds!

e.g. a university a one-man army but an ox an MP (but a Member of Parliament)
a young man a useful thing but an honest woman an heir [the ‘h’ in these words is silent]
a European a horse [the ‘h’ in this word is pronounced]

**Even as plain letters some consonants of the English alphabet take “an” before them because they have the vowel sound in them:

an A a J an S
a B a K a T
a C an L a U
a D an M a V
an E an N a W
an F an O an X
a G a P a Y
an H a Q a Z
an I an R

The article ‘the’ (with an ‘e’ sound) or ‘the’ (with an ‘a’ sound)?

The article ‘the’ is pronounced with an ‘e’ sound when used before words beginning with a vowel having vowel sound, and with an ‘a’ sound when used before words beginning with a consonant having consonant sound:

e.g. the ice age the egg-case but the beach the school
(‘e’ sound) (‘e’ sound” (‘a’ sound) (‘a’ sound)