Why you use the and tha?

Answer 1

"The" is the "definite article, and comes before nouns (eg the word). "Tha" is generally regarded as a slang word, or dialect, possibly representing any of "the", "thee" , "thou", "thy" or "that" depending on local usage.

Answer 2

"The" is used before a noun to specify definiteness, indicating that the noun is known to the reader or listener. "A" or "an" is used before a noun to indicate indefiniteness, implying that the noun is not specific or known to the reader or listener.