As always, when I come across the odd little gramamar points, I try to note them down to better remember them. Here is another one about the subject marking particle:
When writing a sentence, which in English would contain the word when, it is often more natural to use the subject marking particle 이/가 than using the topic marking particle 은/는.
이/가 is therefore often seen with the pattern -(으)ㄹ 때
제 오빠가 덴마크에서 일했을 때, 사무실은 회사 자전거를 가지고 있었어요 ~ when my brother worked in Denmark, his office had a company bike.