Advanced Japanese I: Transitive? Intransitive? Confusing? Yes.
This might be helpful... In short it's transitive vs intransitive.
Here is an exemple I found :
I will start the class = Watashi wa kurasu o hajimemasu
-> Here it's 始める since you took the action to start the class (transitive)
The class will start = kurasu ga hajimarimasu
-> Here it's 始まる since no one took the action to start the class, the class just "starts", so it's intransitive.
The link I gave you should clarify the difference between in/transitive...