Quote: "But is it possible to write an entire program on Dark Basic Pro with just one file? "
Yes - a lot of people do.
When you've been programming a while you find that all your programs can have the same bits of code in them - like joystick/mouse routines etc.
Writing them just once and putting them in separate dba files then telling the project manager to lump them all together when the program is compiled means less work.
All you do is just copy the required .dba files into your current project folder when you start.
If you just have a single .dba file you have to either re-write this code again and again for every program you write, or copy/paste it in from another program.
TDK