Bro simple "a" and an" a is use for comman nouns but "an" is just use before vowels(a,e,i,o,u).For instance: 1."a" book,"a guitar","a pen" etc 2."an" hour, "an"accident", "an" only child etc

oh thanks for replaying.....................

a is used with nouns starting with a consonant sound (a book)and an is used with nouns starting with a vowel sound (an apple) don't make the mistake of focusing on the first letter because for example university starts with the letter u, but the sound is yu so we use the indefinite article a - a university