What happens to teeth if some one drink water at once having hot tea?

Probably nothing: I think what you are trying to ask is what happens to the teeth when you drink something cold immediately after something hot. If that is the case then nothing will happen to your teeth. However if you have any cracks in your teeth or very large silver fillings then going from one temp extreme to the next may cause cracks to worsen or develop in excessively filled teeth.

Drinking and teeth: Shouldnt cause any issues.

Water after hot tea: It depends on the difference in temperature between the two. If the water is very cold, the temperature change will induce stresses inside the tooth, this can result in fractures in the enamel that run from the end of the tooth to the gum. These look like the cracks in a marble if you would heat a marble up and throw it in cold water.