Can't slice a face (and other problems)

So my main question first:

I had a cube, and on the top face I used the knife to cut a “square hole” in the top, so you could look into the cube. Now I can’t use my {loop cut} on the top anymore… why?

Second question: Is using “F” to add faces a bad idea? it seems more often then not it messes something up, not sure if this is standard practice or not.

Thanks for the help!

You’ve got to have a loop to cut. If the loop hits a triangle or an ngon the loop stops so the loop cut will stop at this point.

Second question: Is using “F” to add faces a bad idea? it seems more often then not it messes something up, not sure if this is standard practice or not.
F makes faces, nothing wrong with that, that’s what it’s there for. If you get any shading issues you can just recalculate the normals (Ctrl+N)