This error is so common that it no longer sounds like an error. But, it is!
You may ask what is wrong with saying ”The reason why”. Well, that’s pretty simple – REDUNDANCY!
The word ”why” is a special pronoun that expresses reason. Therefore, saying ”the reason why” is as good as repeating ”reason”. In addition, the pronoun ”why” usually introduces an adverbial clause or phrase, not a noun/nominal phrase. Hence, it cannot serve as the subject of the sentence.
So, instead of saying:
“The reason why she came was to borrow a book.”
“She came to borrow a book.”
“She came because she wanted to borrow a book.”
See you next week!