"The children run to that building."
Translation:아이들이 저 건물로 달립니다.
Children aren't running in the building, they are running to the building
You add 으로 or 로 depending on the end of the word. If it ends with a consonant, you add 으로, if it ends with a vowel, you only add 로. That's why you say 건물로 The 으 practically only exists to make the pronunciation easier and smoother :)
But 건물 ends in a consonant, I thought it should be 으로, not 로. Unless the fact that it ends in ㄹ has something to do with it.
Actually, ~로 is used because 건물 ends with ㄹ.
Eventhough it's a consonant.
We use ~으로 with consonants except the ㄹ.
Because it's referring to "that building" and not "those children". The emphasis is on the building.