Now will focus on some cases

case 1:
extern int i;
int main(){
i=8;
return 0;
}
will not be compiled successfully, because extern variable i is not defined hence there is no memory allocation for this variable and main function tries to change it's value, hence compilation fails here.

case 2:
#include<someotherfile.h>
extern int i;
int main(){
i=8;
return 0;
}
will compile successfully. Though variable i is not defined here, but compiler assumes it's definition in included header file.

case 3:
extern int i=0;
int main(){
i=8;
return 0;
}
will compile successfully, because variable i is defined at the time of declaration hence memory is allocated and main() function can change it's value.