Let us start with the indefinite, they are a and an, the simple rule here is that a is used when the following noun starts with a consonant (any letter excluding vowels), but there are a few exceptions to this rule and the definite article is used when the following noun starts with a vowel (a,e,i,o,u).
Example of an indefinite article:
“I saw an elephant this morning.” ”I ate a banana for lunch.”
Example of a definite article:
the rain, the sun, the wind, the world, the earth, the White House etc..
Articles are also known as Determiners