Then you still don't know which door is the correct one, you've just learned which guard tells the truth and you've used up your one question. The trick is to ask which door the other guard would tell you is the correct one and then go through the other door. If you've asked the lying guard, they'll lie about what the honest one would say and point you towards the wrong door. If you asked the honest one, they'll truthfully tell you what the lying guard would say and also point you towards the wrong door