The difference between the two is that in a dependent event an action will affect the ultimate outcome, but in an independent event an action won't affect the ultimate outcome. A way to tell is if the total number of the "item" at hand will be affected by the action the question is saying. In the case of the example given, that is a dependent event because the total of marbles you started with is changed due to you not replacing the one you first took. Therefore the probability is now different.
