When: -(으)ㄹ 때

-(으)ㄹ 때 means when or while: it turns a verb or adjective into a time clause in front of the noun 때 (time).

• Stem ends in a vowel: -ㄹ 때 (가다 → 갈 때, 심심하다 → 심심할 때)
• Stem ends in a consonant: -을 때 (먹다 → 먹을 때, 있다 → 있을 때)
• Stem ends in ㄹ: just add 때 (놀다 → 놀 때, 만들다 → 만들 때)
• Irregulars: 덥다 → 더울 때, 걷다 → 걸을 때

For a finished action use -았/었을 때: 집에 도착했을 때 아무도 없었어요 (When I got home, nobody was there). Compare 한국에 올 때 (on the way to Korea) with 한국에 왔을 때 (after arriving in Korea). For past states and periods both work: 어릴 때 / 어렸을 때 (when I was young).