I think I understand the basic idea, but I want to make sure I can explain this properly to my students.

I am an Orton-Gillingham tutor and am having trouble with how this is presented and need to firm it up in my mind to explain properly to students.

I have come to understand a bit about the Latin 4 principle parts and how our bases usually come from the 2nd and 4th (right?).

My materials explain that you can drop the -d from suspend and add -tion.  Of course this is incorrect, and I am not sure if this is just supposed to make it easier for the student or a true lack of understanding.  At any rate, I know that suspend comes from the second principal part, while suspense (suspense/ + ion --> suspension) comes from the fourth - of "suspendo"

So, are these what we call "twin bases" or something else?  Do I explain to a 6th grader about the 4 parts, or just talk about the two bases from the same Latin word?  I don't know how deep to go.  Any help from others with past experience appreciated.