Why is there "the" before "inside"?
Good question. Some things, in this case, "the door", have a surface that faces outward, and a surface that faces inward. They have an inside(the "in" side) and an outside (the "out" side). So it makes sense to say the "locked from the inside" in this context as it lets us know that what "side" of the door was locked Saying "the door was locked inside" in this case, makes it sound as if the door is locked inside the room rather than that the door was locked by someone inside the room. Ex: A. The window was broken from the inside! B. Yes. I was kicking my ball in the room.