I'm assuming you're asking if you can do normal phone functions i.e. text, email using Google Cardboard. Cardboard is for demonstrating Virtual Reality, and only apps written for cardboard will display normally in cardboard. If a developer decides to write a Virtual Reality texting app, then yes, you could "write a message using Google Cardboard". However, that really defeats the purpose of virtual reality.
You are unable to interact with the phone screen normally when it's inside the Cardboard goggles.
Cardboard does indeed display normal text. It uses your phone display, so I'm not quite sure what "Can I use Google Cardboard as a normal display?" is supposed to mean.
Have you used cardboard? Do you know what it does? From your questions, I get the feeling you're not really sure.