How to separate opened windows in alt+tab switcher in Ubuntu 18.04?

Go to Settings > Devices > Keyboard and scroll down the list of shortcuts. Alt+Tab will be the shortcut for Switch Applications.

What you want is to set Alt+Tab on Switch Windows instead, so click Switch Applications and press backspace to clear the shortcut for it. Click set. Then click Switch Windows and set Alt+Tab there instead. This will give you the desired function.

It's very simple to solve your problem without change the native keybindings.

All you need to do is type Alt+` (backtick, the key above Tab) if you want switch between the same apps.

To switch different apps, Alt+Tab.

Graphically using dconf-editor

From : https://superuser.com/a/860001/718726 (pasted here because there is no equivalent answer in here)

• Open dconf-editor
• Go to org/gnome/desktop/wm/keybindings
• Move the value '<Alt>Tab' from switch-applications to switch-windows
• Optionally move '<Shift><Alt>Tab' from switch-applications-backward to switch-windows-backward
• If you want switch-windows to work across desktops, not just in the current desktop, you can also uncheck org/gnome/shell/window-switcher/current-workspace-only (Courtesy of @CharlBotha)
• Close dconf-editor
• Press Alt+F2, then type r to restart Gnome.

The last step does not always appear to be necessary, but it should not hurt (especially since it does not close any of your running applications).

Equivalent using command-line (no need to install dconf-editor)

Run the following lines in your terminal:

gsettings set org.gnome.desktop.wm.keybindings switch-windows "['<Alt>Tab']"
gsettings set org.gnome.desktop.wm.keybindings switch-windows-backward "['<Shift><Alt>Tab']"
gsettings set org.gnome.desktop.wm.keybindings switch-applications "['<Super>Tab']"
gsettings set org.gnome.desktop.wm.keybindings switch-applications-backward "['<Shift><Super>Tab']"

If you want switch-windows to work across desktops, not just in the current desktop, you can also execute:

gsettings set org.gnome.shell.window-switcher current-workspace-only false

If you don't notice any change you can press Alt+F2, then type r to restart Gnome.