It is not as easy as it seems to draw a human eye but after some time you will be able to draw it perfect. One of the most important things is that a eye is wet. There is alway a little bit water on it and you have to draw the reflektion of the sun to bring it to live. This little white point shouldn't be too big because otherwise the eye looks liveless. This is the same when you draw the ... to big. The ... should have one third of the eye.