sun give us light from many years so it become now cool but not,why!?

Answer 1

I could write a big answer but why bother when there are lots of web sites around to save the the trouble. In a nutshell, the sun produces light by converting hydrogen into helium through a process called fusion. And there is a lot of hyrdrogen in the sun. See the sources for details of the history of the sun.

Answer 2

there is no reason which can say it becomes cooler the sun's light comes from fusion hence the fusion needs very very few mass to convert it to the energy so the sun has enough mass to remain approximately constant during its life time
of course the rate of fusion is low (in the sun's measures)

Answer 3

It will become cool in time. It is very big, and it is being used up more slowly than you think. It will last not thousands of years, or millions of years, but many many thousands of millions of years. That is maybe hard to believe, but people cleverer than you or me have done the sums.