New to working with Layers — Help!

I'm developing a new employee orientation course in Storyline 360. I'm explaining terminology on a form using layers. 

I'm on the last layer and I'm trying to link the learner back to the base layer with a trigger. However, I don't see the base layer in the trigger list. Am I thinking about this incorrectly, or should I go about linking the last layer to the base in another way?

The only way I can think to do this is to copy the base layer and then link users back to the copy. It works, however, I don't understand why I can't create a trigger to the original layer.

Boy I hope this makes sense.

Please help — I feel like I spend too much time on little things.