-더라고 refers to a recollection of the past. It is something you directly saw, heard or experienced in the past. 고 can be removed from both patterns. This has an effect of reducing the sense of emphasis. In 존댓말 it is always 더라고요. Koreans often type 더라구 but “technically” this is not correct.
From my experience the best way to express this meaning in English is “I saw that”, “I observed that” or “I experienced that”. In the sample sentences below I will simply write say in bold to indicate that depending on the situation the applicable word can be different. Some sample sentences to demonstrate:
- 날씨가 더워서 창문을 열어 두니까 먼지가 많이 들어오더라고요. The weather was hot so I opened the window and then saw that a lot of dust was coming in. This ending of this sentence can be replaced by “…많이 들어오고 있었어요” with no major functional change in meaning. Both are taking about a past action. But the difference when you use 더라고요 is that you are emphasising that you saw it happening.
- 곡성를 봤는데 너무 무섭더라고요. I saw 곡성 and saw that it was really scary.
- 일본에 도착하니까 눈이 오더라고요. I arrived in Japan and saw that it was snowing. This is a link to the -(으)니까 grammar explanation in case you need a refresher or explanation on that.
- 나는 우리 엄마한테 말을 예의 없이 대들었더니 엄마가 서운해하시더라. I talked rudely back to my mum and saw that she was upset.
There are some rules to keep in mind:
- 나는 해외로 여행을 가더라고요 (X). You can’t use this when you are talking about yourself. It doesn’t make sense to say that you saw yourself going on an overseas holiday.
- 제 고양은 강이 많더라고요 (X). You can’t use this when talking about something you already knew from before.
- This grammar can be used in first person if you are expressing your mood, emotion or feeling. For example my 곡성 example above. You can use this in third person as well but you have to change the ending to 무서워하더라고요.
The difference is quite simple. If what you were witnessing at that past point in time had fully completed then you use this pattern instead.
- 일본에 도착하니까 눈이 왔더라고요. When I arrived in Japan I saw that it had snowed. The snow has finished and is no longer snowing.
- 부엌에 돌아오니까 요리가 탔더라고요. When I came back to the kitchen I saw that the food was burnt. It’s no longer in the process of getting burnt, it is already completely burnt. If it was still getting burnt you could use 더라고요.
- 그가 책을 제 책상에 놓고 갔더라고요. I saw that he left the book on my desk. Because its the 았더라 version, you didn’t see him in the act of leaving the book, you only saw that the book was there but you surmised what had happened.