- From the ready stance, Ga is the direction straight in front of you. (Helpful neumonic: "ga" - go, the direction you'll be going in during this form)
- Na is the direction directly behind you. (Helpful mnemonic: "na" is on your backside, your nethers.)
- Da is the direction on your left side.
- Ra is the direction on your right side. (Helpful mnemonic: "ra" is right.)
Why are these directions used instead of "left" and "right"? The idea of Ga Na Da Ra is that these directions are independent of your current orientation. In other words, Da is always the left direction determined by the initial ready stance; Da is always that direction no matter where you are in the form.
For Taegeuk forms, the terms Da 1, Da 2, Da 3, etc. refer to the Da and Ra directions on each of the three lines of the form.
In Karate, the "lines" of a form are sometimes called embusen, and you will sometimes see this term used in taekwondo as well.