Questions about fork Functions

For more information about fork functions-general Linux technology-Linux programming and kernel, see the following. Hello everyone!
I wrote a fork function a few days ago. The book says that fork is the function returned twice. I don't quite understand it.
Write the following code:
# Include
# Include
# Include
# Include

Int main (void)
{
Pid_t pid;

Pid = fork ();

If (pid <0)
{
Printf ("Fork Error \ n ");
Exit (1 );
}
Else if (pid = 0)
{
Printf ("Child Process is printing \ n ");
}
Else
{
Printf ("Parent Process is printing \ n ");
}

// Return EXIT_SUCCESS;
Exit (0 );
}

The debugging is as follows:
(Gdb) break main
Breakpoint 1 at 0x80483f5: file fork. c, line 10.
(Gdb) s
The program is not being run.
(Gdb) run
Starting program:/home/renwei/fork

Breakpoint 1, main () at fork. c: 10
10 pid = fork ();
(Gdb) display pid
1: pid = 3171264
(Gdb) s
Child Process is printing
12 if (pid <0)
1: pid = 29603
(Gdb) s
Parent Process is printing

Program exited normally.

The first question I want to ask is: how is the condition pid = 0 executed?
The second problem is: The book says that the child process derived from the fork function has nothing to do with the parent process. The first return of the parent process or the first return of the child process may occur. Therefore, when running this program, the displayed result may be
Child Process is printing
Parent Process is printing
It may also be
Parent Process is printing
Child Process is printing
.
But what I always see at runtime is
Child Process is printing
Parent Process is printing
This result.
Why?

Thank you!