Bianca S. answered 07/23/19
Native-born French Speaker and French Education
I believe most Quebecers add the "tu" because it became part of their dialect. If you decide to use the more formal and correct way e.g. "as-tu un crayon?" it is perfectly fine, simply more proper. If you want a general rule of when to use the extra "tu", every question that starts with a verb with a vowel, add "t'". For instance, t'écoutes-tu la télévision? t'aimererais-tu aller là-bas? t'oserais-tu faire ça?