Hi, i am from india. when can we take off the mask. we discovered vaccine against corona but things are not over still i guess. why is that? this virus can change its properties and attack again?

Consider some facts. The virus will likely circulate the globe for years and may have important mutations that limit the value of some vaccines to prevent disease or spreader state. If fully vaccinated your risk is less but not zero. A mask provided minimal but some benefit to you & others but demonstrates adult behavior to kids/the less concerned. Time will tell what a\our new normal will be.