Breakdown of chinguga i chaegeun soseolchaegirago haesseoyo.
Questions & Answers about chinguga i chaegeun soseolchaegirago haesseoyo.
Because the sentence has two layers:
- 친구가 = the main subject of the whole sentence: the friend
- 이 책은 소설책이라고 = the content of what the friend said
So the structure is basically:
- 친구가 ... 했어요 = A friend said ...
- Inside what was said: 이 책은 소설책이다 = This book is a novel
In other words, 가 marks who did the saying, and 은 marks the topic inside the quoted statement.
Using 가 here puts focus on the friend as the one who said it.
- 친구가 ... 했어요 = My friend / a friend said ...
- 친구는 ... 했어요 would sound more like as for my friend, they said ..., possibly contrasting the friend with someone else
So 친구가 is the most neutral and natural choice if you are simply reporting who said it.