How is tea, coffee , tobbacco are harmful for teeth?

Each different: Tobacco is the only one specifically harmful. The smoke from tobacco contains toxins that are very harmful to gums by drying them out and actually will cause cellular changes over time. The particles on the smoke stick to teeth allowing debris, plaque and stains to adhere making it harder to keep them clean. Coffee and tea are stain teeth, but are otherwise not particularly harmful.

Staining: Only harm with regards to tea and coffee is that they stain and darken your teeth. There is no particular harm to the teeth themselves.