Q: I compiled my C program into an executable file called test, but I can't run it. Why?

The reason is that test is a UNIX built-in command. When you type test you are running this UNIX command instead of your executable program!

To run the program, prefix it with ./, i.e., type this instead:
   ./test

make is another name that you should avoid.

Bottom line: Avoid naming your executable files with names that are UNIX commands. (If your program can't run, check that it doesn't have the same name as a UNIX command.)