"저는 남자아이를 보아요."
Translation:I look at the boy.
I'm no expert but in a different comment section I read that 를 means 'a' - it simply indicates that you're talking about a specific. I believe 는 means either plural or very broad particular - 남자는, for example, would pretty much be 'men'. And 가 is specific - 여자가 is 'the woman' as in 'the woman over there'. That's my understanding of it anyway. Hopefully it makes a little sense to you?