¿Por qué tú quieres hablar español?
This translates to "Why do you want to speak Spanish?"
Here's a breakdown of the corrections:
* ¿Por qué? is the correct way to ask "Why?" in Spanish.
* Tú is the correct form of "you" in this context.
* Hablar is the verb "to speak."
* Español is the noun "Spanish."
The phrase is now grammatically correct and conveys the intended meaning.