Here's what I need to do:
If someone selects q1_A1, then q2_A1 will be selected. q2 is a hidden question on the next page. Both are List Radio questions.

I've looked at other forum replies, tried multiple expressions, and read all the documentation but can't seem to figure out how to do this.

Equation question type just after q1

{q2=(if(q1=="A1","A1","")}

THANK YOU! That worked for the List Radio, but what if q1 is a Multiple Choice type question? In this case:
If the first checkbox of q1 is selected (regardless of whether any of the other checkboxes are selected or not), the first option of the List Radio question will be selected. I tried the same equation but it doesn't work...

q1_A1 == "Y" or !is_empty(q1_A1)

www.limesurvey.org/manual/Expression_Man...code_Variable_Naming