Question

When do you have to add or multiply in probability

Answer 1

In probability, you multiply when you want two or more different things to happen at the same time.  You add probabilities when the events you are thinking about are alternatives, which means they are NOT happening at the same time. 

Answer 2

When the question asks for the probability of one condition AND another condition, you multiply

When the question asks for the probability of one condition OR another condition, you add

Hope this helps