China DOS Union

I have 2 questions to ask the experts here,
1 I'm a beginner in C. Today I wrote a program for an exam. When debugging it, there are no syntax errors anymore,
but it still never outputs the correct value.
Could you please point it out for me—where exactly is the mistake?
main()
{int a;
int i=0;
long sum=0;
for (;i<3;i++);
{ scanf("%d",&a);
sum+=a;
}
printf("%d\n",sum);
}
(Input an array and output the sum of the digits)
2 I'm using the XP operating system. When I use TC from the command prompt, why is it that when I switch to drive D (I installed TC on drive D), I can open it, but when executing a program it always can't open the header files, while if I install TC on drive C and run it, this problem does not occur? What's going on?
Thanks for your advice!

The path is probably not set correctly. Go into Option-------->Directories and take a look.

Then what about the program above? Where exactly is the mistake?

Man, it's a very simple mistake, you're just not careful enough. It's in these two lines, you should be able to find it yourself

for (;i<3;i++);

printf("%d\n",sum);

Besides, this isn't really the place to discuss the C language, so not many people will answer your question.