Subject vs. Object

Today in class we discussed subject pronouns and objective pronouns,and how they are used. It can be very confusing at times because I normally base it off of the way that I speak.I have been told that is not the case for ACT and other major tests. I feel that I will do fairly well on these parts of the test considering how good I did on the practice questions we did today. The only downfall is that I can not explain how I get my answers.