How do i removed the Curse don't know where i can restore my health from it.
Are you referring to the reduction in health when you die? If you are, you have to use a human effigy to restore back to your human form and have access to your full life bar. If you remain a hollow (dead) and you keep dying, your health will decrease little by little. The reduction will usually cap at 50% of your full lifebar. I've heard that if you sin, it could possibly reduce below the the 50% cap.
So yes. Using a human Effigy will solve that problem. if you don't have one, look for the ring of binding. It will increase you lifebar while you are hollow. That might give you a better edge until you find an effigy. Or you can help defeat a boss in coop. There's a lot of talk about this being a bug, but it worked for me.
...what do curse and petrify do in this game? I think I saw some guy on a staircase who had been petrified (instant kill?) and I broke him for modesty's sake, but I have no idea what curse does.
Also, I'm not 100% on bleed and poison either... set % of HP instantly in the first case I presume, but how does Poison BNS work? Does it increase the amount applied to the bar, the rate of HP decrease, the duration of the effect, or some combination of the three?
How do I open the door near the fourth bonfire? As far as I can tell you only need to be undead to open it, I went there straight after beating the Demon of Song boss while undead was able to open it. My brother who was further ahead in the game went back and couldn't open it as human, so he died and came back and violá.
I made a mistake of attacking the ancient dragon before getting the ashen mist heart, am I screwed or do I get it if I beat him. Also is there a way to reset the ancient dragon to talk again....help pls