Normally I'd highlight an email and it would load underneath... however now I have to double click on the email and it opens in a separate window... what happened and how do i fix this?
Do you have your mail set to display a 3-pane view rather than 2-pane?
You just grab onto the bottom of the inbox window and drag up to display as 3-panes.
You turned off the preview pane. There should be a dot down at the bottom of the main Mail window with your messages in it. Drag it to the size you'd like your preview pane to be.