There are three kinds of assimilation. The most common is regressive assimilation in which the first sound of the second word, affects the last sound of the first word. Then there is coalescent assimilation, which is a type of regressive assimilation. Thirdly there is the least common but also important type of assimilation known as progressive assimilation in which the last sound of the first word affects the first sound of the second word. After doing the exercises, the differences between the three should be clear.