If you threw a knife at the door, and it stopped and hit the ground before it reached the door, the knife would be in front of the door (location). If you threw the knife so hard that it hit the door, it would be stuck in the front of the door. (as a result of an action). If you threw the knife as in the first example and someone asked you where their knife was, you could say it's in front of the door, or it's AT the front of the door - both describe a location.

"What IS THE difference between 'IN FRONT of the door' and 'IN THE FRONT of the door'?" "The box is IN FRONT of the door" is how you would usually say it. (the direction of; opposite of "in back") "I'll be waiting for you IN FRONT." (the direction of) "Your pen is right IN FRONT of you." (the direction of) "The box is IN THE FRONT...of the door" is what you would say for THE PLACE OR THE LOCATION of something. You add "...of the door" to make what you said clearer. "I'll be waiting for you IN THE FRONT of the building." (the location of) However, for me, there isn't a big difference between saying "I'll be waiting for you IN FRONT of the building" (the direction of) and "I'll be waiting for you IN THE FRONT of the building" (the location of). Either way I know you won't be IN BACK (the direction of).