In Yahoo! Mail, the preview window no longer displays by default. You can easily change this setting, though, if you prefer to have the preview window display in your inbox. There are two places that you can change this setting:

From your inbox:

1. At the top of your inbox, click on the "switch view" icon (this is located between the folder icon and the gear icon).
2. Under "Show Messages" select In a Scrolling List (with Preview).

From the "Options" menu:

1. Sign in to Yahoo! Mail
2. Click the Options menu in the upper-left corner, and select Mail Options from the pull-down list.
3. On the left side of the page, click on General Options
4. Under "Messages settings" select Show all messages in a scrolling list (with Preview Pane option)
5. Click Save changes in the upper left corner.
6. Back in your Inbox view, if you still don't see your preview pane, click the Show view button in the toolbar.

The setting "Show all messages in a scrolling list" means that all of your messages will appear on a single page and you can view them by scrolling down. In addition, when you select this setting, the inbox and the left navigation bar are treated as separate panes, and you can scroll down each individually.