[warmare (OP)]

A man sinks in water if he doesn't know how to swim, but a person knows swimming doesn't sink into water.
 What is the reason behind that?
 In which ways swimming helps a person not to sink ?

[syhprum]

when we learn to swim we acquire two skills,firstly to push ourselves along to keep our head out of water and secondly to gulp in air when our head is clear of the water and never swallow any.
the body has little or no buoyancy in fresh water and unless we make an effort we tend to sink but in sea water we have less tendency to sink but the effects of swallowing water are much worse.

[Soul Surfer]

Most people are slightly positively buoyant with their lungs full. you can test this in deepish water by standing straight up in the water like a hydrometer and see where you settle down its usually somewhere between your eyes and your chin if you are higher this means you probably have an excess of fat if you sink you have very little fat.  swimming or treading water gently will easily keep you afloat to breathe.

If you fully deflate your lungs you should sink.  If you don't again you probably have a excess of fat. 

[Geezer]

Most people are slightly positively buoyant with their lungs full. you can test this in deepish water by standing straight up in the water like a hydrometer and see where you settle down its usually somewhere between your eyes and your chin if you are higher this means you probably have an excess of fat if you sink you have very little fat.  swimming or treading water gently will easily keep you afloat to breathe.

If you fully deflate your lungs you should sink.  If you don't again you probably have a excess of fat. 

Right on. When you exhale, you sink. I have conducted this experiment often.