을 때 is used to say 'when I do V'. It is used in past and future tenses; note that in the past tense 았/었 must be added to the verb:
여행했을 때 저는 많이 친절한 사람을 만났어요.
When I was travelling I met a lot of kind people.
일번에 있을 때 저는 오사카에 방문할 거에요.
When I'm in Japan I will visit Osaka.
학교에 다녔을 때 이 학과를 공부하지 않았어요.
When I was in school we didn't learn those subjects.
런던에 살았을 때 저는 항상 돈을 없었던 것 같아요.
When I lived in London it always seemed like I had no money.
대학교를 졸업할 때 뭐 할게요?
When you graduate from university, what will you do?
신천역에서는 도착할 때 열차를 내리고 3출구에 기다리세요.
When you arrive at Sincheon station, get off the train and wait at exit 3.
을 때마다 is used to say 'whenever I do V':
밥을 먹을 때마다 먼저 물 한장 마셔요.
Whenever I eat dinner, I drink a glass of water first.
옷을 사러 백화점에 갈 때마다 큰 사이즈는 없어요.
Whenever I go to the department store to buy clothes, they don't have big sizes.
고속버스로 탈 때마다 저는 안전벨트를 꼭 매요.
Whenever I travel by express bus, I fasten my safety belt securely.
술을 마실 때마다 저는 더 졸려요.
Whenever I drink alcohol I'm more sleepy.
어휘
매다 -- to fasten (a belt)
크다 -- to be big
술을 마시다 -- to drink alcohol
졸리다 -- to be sleepy
안전 -- safe
꼭 -- firmly, surely
학과 -- school subject