The instructions from https://formidableforms.com/how-to-create-inline-form-wordpress/ do not seem to be current.    

It does not work when following it. Additionally I tried the class frm_half instead of frm6 - still does not work.

With Submit button mode in "inline":     

If I set the email form field as frm_half, the outcome is this that the submit button is truncated down and even the full Submit word is not visible.    

If I set the email form without any size class it looks like the form applies some kind of padding on the sides making it impossible to work with in the layout of my site.