Ive got the harry potter and the deathly hallows part 1. And I'm in the..

Generally, when there's a gete that need to be opened, there's a green switch somewhere else that you have to turn on. It may be that you just have to look in the surrounding rooms and corridors for the right switch. If the switch is on the other side of the gate that you need to open, then look at the fence that's seperating you from where you want to go. There should be a break in it, that harry is too big to get through. This is for a decoy detonator. Activate a decoy detonator (if you don't have one on you, destroy some of the furniture nearby; there should be one under that), and draw the blue line so that it goes through the hole in the fence, and ends up next to the switch that you want to turn on. This should activate the switch, and open the gate. Hope that helped!