what happens for return 1;

Answer 1

If you're talking about main, when the return is different from 0, it means an error occured. If something happens, and the program has to stop running, then it returns a non-zero value.

Answer 2

makes sense so if it ain't zero an error occurred and it has to keep running thanks

Answer 3

there is no difference whether you write return 0 or return 1.the program will run successfully. You can even write return 2 or return 7 whatever you want.