Here is how to enable a preview pane in Gmail
Finally, a preview pane (similar to a vertical view in an email client) is available using the web interface for Gmail. If you’re familiar with Gmail settings, you can enable the preview pane in Settings/Labs/Preview Pane, otherwise step by step directions are in this article. The Labs section of Gmail is where various experimental Gmail features can be enabled or disabled. Many have come and gone over the years, hopefully this one will stick around.