Question

why hydrochloric acid secreted in our stomach does not harm us??

Answer 1

the Linings in stomach is coated by mucus which protect its from hydrochloric acid

Answer 2

The lining of the stomach is coated with mucus to protect it from acid damage. Moreover, the stomach protects itself  from its own enzymes, or burnt by the corrosive hydrochloric acid, by secreting sticky a bicarbonate rich solution on mucus that clings to the stomach walls. Bicarbonate is alkaline, a base, and neutralizes the acid secreted by the parietal cells, producing water in the process.

If this layer becomes damaged in any way it can result in painful and unpleasant stomach ulcers.