How do you know when to add probabilities together and when to multiply them together

A probability that an event occurs will always range between 0 and 1. If you want to know the probability of two events both happening you would multiply them together. For exampke what is the probability I flip a coin and it is heads twice in a row. The probability of my getting heads in the first spin is 1/2. The probability of me getting heads in the second spin is also 1/2. Therefore the probability of me getting heads twice in a row is 1/2 X 1/2 which  equals 1/4. 

Probabilities are added together when you want to know the probabilty of an event occuring OR another event occuring. For example if I roll a die and want to know the probability of  getting a 2 or a 6, I would add the individual probabilities of getting each of these. The probability of rolling a 2 is 1/6 and the probability of rolling a 6 is also 1/6. Therefore the probability of getting a 2 or a 6 is 1/6 + 1/6 which equals 2/6. This is the same as 1/3.