I just finished baking a cake for my niece and nephew and in the center of the cake is hard...I waited 40 minutes to open the oven and i noticed it was not done yet...is it because I opened the oven to soon?

if its over baked the outside edges will get hard.
if its underbaked it will generally sink in the middle, and you do get a funny looking bit cake, which is very dense, so maybe that could be the problem here?
xx

I know what you mean. Is it like a hard rubber like cake in the middle?