Normally when in the non-classic view in Mail there is list on the left for my mailbox, a list in the middle for my inbox, and a list on the right for the messages contained in a conversation. I don't see the last list on the right. Is there a way to get that pane back, perhaps through an option in the menu? I'm running the last versions of everything (10.11).
I think you've dragged the right edge of the middle list all the way to the right, which then hides the preview of the messages. If you hover your mouse of the right edge of the scroll bar on the right you should get the draggable crosshair mouse pointer. Hold it and drag left and you should get the previews back.