When you multiply is when your cases are dependent. From your example, you multiply because if Denice scores a goal and Jacqueline scores a goal, it counts. But if Denice scores and Jacqueline doesn't, it doesn't count. You multiply because Denice is depending on Jacqueline to score a goal.
Another example would be rolling dice. Assume you want to get a 6 from both die. To get a 6 would be 1-5, 2-4, and 3-3. There are two separate dies to consider. If you roll a one, that die is depending on the other die to get a five. I hope I explained well.
You add when the cases are independent. Your example isn't a very good one(no offense) because you also have to consider if Greta's mom takes her shopping, which is a dependent case. Anyways, skip that and go to the ice cream shop. They aren't depending on each other. If Greta goes by herself, then you don't have to take if Greta's mom took her into considering, because Greta is all by herself.
I know I'm not the best explainer but I hope you understand!
