How can I attach this hand? How do I successfully attach this hand I made to my forearm, without creating extra geometry. I just can not seem to connect them properly, and when I subdivide the surface or knife bisected the lines to create extra vertex points, it messed up my faces on the rest of my forearm and wrist area.
You need to reduce your topology to be able to make the joint. Try to avoid triangles because it can create ugly artefacts. Typically you can use the method above, but it won't work every time. Here a site about topology. Maybe share your file here so that we can test some solutions?