Dogs have a hollow nail and if it is allowed to grow very long, there is a nerve and vein that is present in the nail that will cause the dog pain if you cut it. They will also bleed excessively. The nails should be kept short in order to avoid this. If you have let them get too long, you will have to trim them over the course of several weeks at progressively shorter lengths till they are the right length. You can also take them to a vet and let the vet sedate the dog and then cut them to the right length and apply a steptic to stop the bleeding.