You can do ifs, elses, ... and whatever you could do in JS in logic. Example with some comments
If you're having trouble about picking, it's not exactly the same. By default (when no condition is applied), every object in the layout is picked. After each condition, you select a little more elements, removing from the selected list elements that don't meet the conditions. That means that adding a condition "Ennemy.HP > 0" won't pick ennemy with HP>0 but will remove every ennemy with HP <= 0 from the selected list. This is why there are conditions like "pick all", to add items to the selected list.
Certain conditions, like "compare 2 values" in the System object, are boolean conditions, and won't be considered as affecting an object type. They will just block an event from triggering if they are false.
The best way to really understand all the possibilities of this system is to create a sample capx, and try with lots of debug, see what triggers in what cases, find out what the "wait" action can do to the function parameters, discover that removing every object from the selected list will prevent the event from triggering, thus will trigger the else event, ...
Experimentation is a nice friend here, but there are also lots of things in tutorials ;)