Sometimes a Mac app acts up and you have to kill it. Well, not actually kill it, but you may have to force it to quit so you can relaunch it. Here’s how you use Force Quit in macOS Sierra:
Choose Force Quit from the Apple () menu, or press Command-Option-Esc.
- (This is similar to pressing Control-Alt-Delete on a PC.)
- Select the app in the Force Quit window, then click Force Quit.
Of course, you should try quitting it in the normal way first by choosing Quit from the app's menu in the menu bar, or press Command (⌘)- .
(This how-to is based on my experiences and info on Apple's support pages — where the images come from.)