how do i move preview pane in apple mail High Sierra OS

Hi Drubnitz,

Thanks for reaching out to Apple Support Communities.

You'd like to move the preview pane in Apple Mail.

We're happy to assist.

We recommend checking out the steps in the section titled "Change how emails are shown" from the article here: Read emails in Mail on Mac - Apple Support

In the Mail app  on your Mac, do any of the following:

*Turn off conversations to ungroup messages: Choose View > Organize by Conversation. Email conversations are off if there’s no checkmark next to the command.

*Sort messages: Choose View > Sort By, then choose an attribute, such as From or Size, and a sort order.

*Show the message preview on the side or bottom of the Mail window: Choose View > Show Side Preview or View > Show Bottom Preview. 

*Show more details: Choose View, then choose details to show, such as Date and Time or Message Size (a checkmark indicates a detail is shown).

*Show messages with dark backgrounds (when using Dark Mode on your Mac): Choose Mail > Settings, click Viewing, then select “Use dark backgrounds for messages.”

*Resize the message list: Drag the bar that separates the message list and the message preview to the left or right.

Alternatively, you can use column layout to view messages. See Use column layout.

We're recommending the steps above because they directly address the issue you initially posted about: you'd like to move the preview pane in Apple Mail.

Please let us know the outcome after giving the above steps a try.

Cheers!