In blender, after I press F12 to render, the view changes. Is there a button or key that I can press to go back to the 3d view?
Pressing Esc should do this by default and in addition to WChargin's answer, another way about this is to go to the Render context in the Properties panel and under Render, change Display to New Window. This will have your renders appear in a separate window, one that you can close without exiting Blender or switching views.
I assume you're asking about returning to the model view after seeing a render.
Usually, pressing Esc will do the trick.
If this doesn't work, try F11, or Window > Show/Hide Render View.
If this still doesn't work, you can do the following:
- Save the file.
- Open the default file (Ctrl+N for a new file).
Open the old file without the Load UI setting checked in the Open Blender File window. This will reload the file with the default user interface: