A and AN are indefinite articles. A cat, a bicycle.. The is a definite article. The cat, the bicycle.. A definite thing is something that is definitely it, where as an indefinite article could be it.

"an" is used before vowels :a,e,i,o,u an apple , an orange , an elephant a and an are for indefinate singulars. for plurals you add "s" "I like owls" "I ate an apple" "I ate the apple that was on the table" (specific)