My tooth hurts and has some black line at the bottom? how to cure it? thanks

Tooth pain: The black line is probably just staining of the tooth/enamel. The pain though can be sign of tooth decay or infection, I would see your dentist.

Dental exam: The first step is to see a dentist and find out exactly what is causing your pain. The black marks on your tooth could be decay, stain, or tarter. Treatment is different for each of theses. This will not get better on its own. See a dentist

See dentist: See your dentist, let the dentist evaluate the tooth both clinically and radiographically. It may be as simple as cleaning off calculus, which is black or there may be caries. Let the dentist evaluate.

See a dentist: The black line can be a cavity, calculus, or even staining in an area of abrasion. You'll need a dentist to diagnose and restore the tooth. All the best...

With a telephone: This isn't a DIY project. You need professional help. Please call your Dentist. May me stain, tarter, fracture, or infection (cavity). Pain is an indication of a serious problem. Without treatment this will only get worse. Please call your Dentist for caring qualified help. Good luck.