When I run water anywhere in the house the toilet in the bath room starts bubbling. Then if you flush the toilet it gets backed up and will not flush, but it still goes down very slowly.as the water goes down I can hear water pooring out under my house, because there is a vent in our bathroom, when you take the grate out of floor you can actually see the water. What's causeing it and how do I fix it...

I'm betting your house sewer is plugged and needs to be augered. The water running under the floor is probably due to a leak in the drainage system, most likely from the wax ring being blown out by the pressure of a backed up system, or any leaky area in the drainage system. Find the area of the leak first, and make sure to fix it, then auger the main drain. Please let us know what you find.