situation

We all know that past tense indicates something happened in the past, but why sometimes or most of the time, I notice my history teacher writes things in present tense on the blackboard. For example, Civilians suffer a lot in that period. King doesn't listen to people's complaints. He still puts taxes on all the products. How come is that? And there is another thing, sometimes people tell you something that happened in the past, they first tell you in the past tense, then suddenly all present tense. For example, Yesterday when I got home, my sister comes up to me and asks me if I bring her pencils. Then, she sits down and makes no noise.

thanks.