Complex sentence: There was one entry that was written during their trip to Fog Island.Su-Eun planted seeds in the hill during their visit and promised that when those seeds become flowers, she'll come back with Su Ho to that island, year after year, after year.
Split into simpler sentences: There was one entry that was written during their trip to Fog Island. It seems Su-Eun had planted seeds in the hill during their visit and promised herself that when those seeds became flowers, she'll come back with Su Ho to that island, year after year, after year.