し終える is transitive, while 終わる can be both transitive and intransitive. し終える (transitive) it means that you finish doing something completely, and that you finish doing ahead of time. 　例 )　本を読み終える。 I finish reading a book (because I wanted to finish reading it) 　例2) 授業を終える。 I finish a class. (this example implies that I finished the class "ahead of time") ＋　し終える　also implies that you brought good results. 終わる (transitive) you finish something, but it does not mean that you finish doing it completely. 　例 ) 本を読み終わる。　I finish reading a book (I did not have to finish reading it by then) 　例2)　授業を終わる。 I finish a class (because it is time for a break.) ＋　終わる　does not imply a good result. 終わる (intransitive) 　例 ) 授業が終わる。　the class is done.

Let me add my two cents. 終える: put an end x 終わる: come to an end Generally, the subject of 終える is the one who puts an end to something, even though the subject phrase is often omitted. On the other hand, the subject of 終わる is the thing coming to an end. 舞台を終える : the subject is omitted, but it should be someone who works for the play like an actor or director. 舞台が終わる: the subject is the play. It just describes the fact that the play finishes.