NERDTree is a priceless Vim plugin that gives you access to the project directory structure. Some shortcuts are documented when you press ?, but I think this list leaves out a few essential ones:
1. Press Control + ww to navigate between the open buffers. Alternatively, press Control + w and then h j k or l to navigate to your direction of choosing.
2. When you are over a NERDTree file, press i to open a horizontal buffer and s to open a vertical buffer. Press t to open a new tab.
3. To open a blank tab, press Command + Shift + T. I often open a blank tab and use the CtrlP plugin to Command + T to a new file.
4. To open a horizontal buffer from the command line, press :sp path/to/file and use :vsp path/to/file to open a vertical buffer.
5. p takes you to the parent directory and P takes you to the root.
NERDTree improves my Vim programming productivity immensely. Like Vim, I only need to know the basic NERDTree commands for most of the productivity gains.