Hi Denise, sorry you are having problems.

You can not add additional placeholders to a Master Slide, which is why you
have had so much difficulty, However, you can add regular textboxes, So if
you have a title that does not change between slides, you can add the Main
Title as a pre-filled textbox, and use the Title placeholder as your sub
title.  Alternately, you can make a slide layout the way you need and just
copy the slide instead of using the new slide command.  Both of these are a
bit clunky, but will provide a workaround.

To change text levels (with in text boxes, autoshapes, and text placeholders
only, this does not work for title placeholders), you need to let PowerPoint
know which use of the tab key you mean.  PowerPoint assumes (rightly or
wrongly) that the tab key is mainly used as a tab indent, but if you select
the text on the line you want to demote, then press tab, it knows you mean
differently, and will demote the text line one level.  You can un-demote
text by pressing Shft+Tab with the text selected.