the is what we call the definite article which signifies that the noun that one is talking about is already known to the speaker whereas "a" is an indefinite article that is basically the direct opposite of the article"the". Ex. Wanna eat a banana?(not specific about which banana he/she would want to eat and the person who's asking the question is merely inquiring if he/she wants a banana.) Ex. A:which banana are you talking about? B: THE banana that is located second to your left. there could only be one banana to which the person is pertaining .

i have simple idea " a" is add before the nouns are not specifically identified e.q when i say: i ate a banana . you don't know which banana i mean "the" is add before nouns which are known by the speaker and listener e.q thank you for the banana . i talking about specific banana that you gave to me .