The Trash won’t empty
As files you drag to the Trash aren’t immediately deleted, macOS will let you ‘trash’ files that are in use by apps. But when you try to empty the Trash, the system will refuse (pardon the pun), saying the item is in use. If you can guess which app was using the file, quit it.
Still won’t empty? Hold å while doing it (or press ß+å+ç+∫). Restarting your Mac may also help. If it still won’t go, try emptying the Trash in safe mode (see page 24). If the Trash won’t empty because an item is ‘locked’, open the Trash, select the item, press ç+i, untick Locked and retry.