Answer:
Though I am only two words I have thousands of letters in me
I am in the past never in the future. I don't exist but have existed. I saw what you saw and that is what I will ever see.
I have no voice but I can teach you all there is to know. I have spines and hinges but I am not a door. Once I’ve told you all I cannot tell you more.
My days are numbered.
In many hall ways you would stand if not with me in hand.
No matter how little or how much you use me, you change me every month.
What am I?
Animal
Difficult
Easy
Food
Funny
Kids
Logic