아이를 키운다는 건 참 힘든 일이에요 아이를 키우는 건 참 힘든 일이에요 these two sentences are similar but slightly different. the difference between them is revealed if we place '저는' at the start. 저는 아이를 키운다는 건 참 힘든 일이에요 this says 'it's hard for me to raise a child'. but if we do the same for the other sentence, it won't make sense. 저는 아이를 키우는 건 참 힘든 일이에요 this doesn't make sense because the topic particle 는 is marking both 저 and 것 as the topic of the sentence. this second sentence can make sense if we change it to either of the following: 저는 아이를 키우는 것입니다 아이를 키우는건 참 힘든 일이에요 which is the second sentence you originally wrote and this means 'it is difficult to raise a baby'. with "아이를 키운다는 건 참 힘든 일이에요" the 저는 is omitted but its presence is implied. so that first sentence you wrote means 'it is difficult *for me* to raise a baby'.