HOW TO TRIM/EDIT HATCH

I have been trying to, for example trim a hatch corner in a rectangle. I hatch the rectangle, draw a diagonal line in a corner to trim the hatch in the small triangle corner and have not been able to do it. The few posts I've found in the net do not work for me, or I am not understanding them. his should be a very simple procedure to do. So Help, Tnks!