Tapadh leibh vs Tapadh leat
When do you use tapadh leibh and when do you use tapadh leat?
The former is for plural and formal second person, the latter for the familiar second person. I find in the exercises the formal seems to be used for "professor" "father" and "grandfather" if I recall correctly.
you'll see the same with agad/agaibh and thu/sibh, among others.