I guess she asked about simple past and past continuous, not simple past and PRESENT continous. Basically, past continous is used for an action that happened in a period of time, whereas simple past is for an action in a specific time Both are finished.

Simple past: I finish(ed) my book. I (pronoun), to finish (verb with ed) and noun. Present continuous (three different types) I have been finishing my book. (it started in the past and is still happening now) I am finishing my book (what you are doing right now) I am finishing my book later on tonight (might happen in future) Hope that is useful! http://www.englishpage.com/verbpage/presentcontinuous.html