Question

why should we always breathe through the nose and not and not throught the mouth

Answer 1

Our lungs require air which is warm, moist and clean. As the air passes through our nose it is moistened by the slimy mucous present in the nose. Mucous is secreted by the inner lining of the nose. The mucous and the hair present in the nose together trap bacteria and disease causing germs. We would not get clean air if we breathe through our mouth instead of our nose. A number of infections are caused to the teeth is we breathe through our mouth. Therefore we should always breathe through our nose.

Answer 2

the inner lining of our nose is surrounded my mucus and hair which purifies the air from  dust particles and the air will be fit for beathing and the dust wont entrer our respiratory system
but if we breathe through mouth the dust particles may enter our respiratory system which may cause infection
so we should breathe through our nose and not through our mouth