I start with a cube, then go into top view. I'd like to change its shape into something else but when I press G, only the top vertex is grabbed, and it changes my square into some weird triangle. I'd like to know if there is a way, without leaving top view, to grab both the top and bottom vertices of the square. It seems like a common problem and I can't figure it out. Should I instead be starting with a plane then adding thickness later?
Either you go into wireframe mode Z or you click this little button:
This will set the limit selection to invisible too. Then select your vertices with B and drag. But: Be aware of these methods will select ALL vertices inside your selection (e.g. if you´ve a more complex mesh).