Answer:

Nail discoloration - nail disease

crumbling nails - Nail disorder

How to prevent:

To prevent nail damage, don't:

• Bite your fingernails or pick at your cuticles. These habits can damage the nail bed.  
• Pull off hangnails. You might rip live tissue along with the hangnail.  
• Use harsh nail care products. Limit your use of nail polish remover.  
• Ignore problems.

Explanation:

Hope it helps :3