Dear sir, my feet nails has broken and getting black i don't know why. what is the main causes thanks a lot?

Nail problem: Cause could be many like trauma, blood under the nail, fungus or something more serious. See podiatrist for nail biopsy.

Toenails: Sorry that your nails are broken. How did they break? Since you are 18, it is probably from a trauma or injury that they are turning color, however sometimes it could be more sinister. If the color is concerning to you, you may want to see your physician for a referral to a specialist (either podiatrist or dermatologist).