"Force" the menu highlight to show for selected slides?

Hi E-Learning heroes, hoping you can direct me (and that it's simple!):

Per the screen snips/scibbles attached, I have "main" screens that link to other screens (what I think of as 'sub-screens'). When the user is on  the "main", the menu is highlighted as expected. 

How can I keep the highlight showing when the 'sub' page is being viewed?

Thanks in advance!

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