This is the 10756th most frequent Korean word.
귀국했다
"귀국했다" means "returned to one's home country."
Here, '귀국한' is the past participle form modifying the following clause. It implies the action of returning has been completed.
귀국한 후 가족들과 시간을 보냈다.
After returning to the country, he spent time with his family.
In this usage, '귀국할 예정' is a future plan, indicating an intent to return to one's country.
이번 주말에는 일본에서 귀국할 예정입니다.
I am scheduled to return from Japan this weekend.
The word '귀국했다' in this sentence shows a completed action, indicating the subject returned to their country (in this case, Korea).