Emails are stored in folders, so to read them you open a folder, then open the message.

When you first open Yahoo! Mail, a Welcome page opens in the main window. If you have any new messages, the word "Inbox" followed by the number of new messages appears below the Welcome. Click this. Or, you can click "Inbox" at the top of your folder list in the navigation bar to the left. Or you can click any other folder if you're looking for a message somewhere other than your Inbox.

A list of the emails in your Inbox(or the folder you clicked on) opens in the main window. Click once anywhere on the info line (sender, Subject, Date/time) of the message you want to read, and the message appears in the "reading pane" below. This is a preview window. See it? You can also double-click the info line for the message, and the message opens in the entire main window, under a new tab. This gives you more room to read the message, but it also covers up the sub-window that listed the folder contents. Do whichever you prefer!

After you've read a message, the message's info line (sender, Subject, Date/time) no longer appear in bold in the list of messages in the folder. Only the info lines of unread (fresh!) messages appear in bold.