How do I add folder to Ubuntu 18.04.1 Nautilus Bookmark list?

To add a folder to the bookmarks in Files (aka nautilus), either

  • Press Ctrl+d to add the folder you are currently in to the bookmarks list in the left pane (under the horizontal line separating the "special" folders, e.g. "Recent", "Home", etc.
  • Drag a selected folder to the left pane.

You can see all defined hotkeys in Files when you press Shift+?