Did you know that you can get back emails you've already sent if you act quickly enough? If you were paying attention when you sent an email, you might have seen the big "UNDO SEND" button in the list of messages you've already sent.
As soon as you click the "Send" button, all of your emails are sent. On the other hand, Gmail lets you send your emails at a later time. With email scheduling, you can tell Gmail to send your email at a certain time or date in the future. You can only schedule one email at a time, though, so this feature is not very useful.
When you open the reading pane, your inbox is split in two. On the one hand, you'll see a list of your emails. On the other hand, you'll see the email you clicked on. You can also choose where the reading pane will show up.