Why when I import a PDF to CDR, it goes to a new layer "0" and not to "Layer 1" the default?
I can replicate this issue with Autocad and Corel. If I have layers in Autocad (which is always handy) then if I choose the dwg to pdf option in Autocad to print (plot) and the box is ticked that says "Include layer information" then you will get a pdf that contains layers. The default Autocad layer name is 0. You will see these pdf layers in Acrobat as well, when you look at the left hand panel in Acrobat. Some pdf's have layers, most of them don't.
When you import this layered pdf in to Corel, then Corel keeps the same layers. This is quite handy too in Corel, but sometimes depending on the complexity of the file this might not be so good idea. Then you just need to group everything to Layer 1. It is also funny that if you import a layered pdf in to Corel multiple times, it will always create the same layers again. This means that there will be multiple layers with same name.