Student or Learner
I WOULD LIKE TO KNOW WHAT ARE DEFINITE AND INDEFINITE ARTICLES
ON WHICH BASIS WE SAY a and an arew Indefinite and THE is DEFINITE ARTICLE
"The" is used to refer to a definite thing, and "a" is used to refer to something we don't know the exact identity of.
A queen is any woman who happens to be queen.
The Queen of England is a definite person -- there is only one person who fits that description.
Usually, we only use the indefinite article the first time we mention something in conversation. After that, we use the definite article to mean "the thing we have already mentioned".
I can see a cat. [I don't know which cat -- it could be any cat.] The cat is sitting in a tree. [The cat I mentioned in the first sentence.]