Let me five you a little example here. 
 
 
Your friend asks you "what did you do last night?"
and you say " I read a book"
어젯밤 뭐 했어? / 책을 읽었어.
but "책 읽었어" is more natural. 
영화를 볼 거예요.--->영화 볼 거예요.
신발을 샀어. ----> 신발 샀어.
 
I think it's only for the conversation and informal situations.
영화 볼 것입니다. sounds strange a little. 
영화를 볼 것입니다. is better.
신발 샀습니다. --->신발을 샀습니다. 
I just realized this. Thanks to you!!
Omitting the object markers may be in the natural converation.
what do you think? 
