Learn Hardcore Korean: daehoee chamgahal ttae pilsu seoryureul jechulhaeya haeyo. - You must submit the required documents when you participate in the competition.

The particle 에 marks the target or location of an action. In 대회에 참가하다, 에 indicates “in” or “to” the competition—that is, it shows where you participate.

How is 참가할 때 formed, and what does it mean?

참가할 때 means “when (you) participate.” It’s built from the verb 참가하다 (“to participate”) by dropping –다 and adding –ㄹ 때 (the time-when suffix). Since 참가하다 ends in a vowel stem + 하다, you get 참가 + 할 때.

Why isn’t it 참가하는 때 instead of 참가할 때?

For verbs in Korean, the time-when expression uses –ㄹ 때 (future/attributive) rather than –는 때. Using 참가하는 때 would be grammatically odd; 참가할 때 is the standard pattern.

What does 필수 서류 mean, and how are 필수 and 서류 constructed?

필수 서류 means “required documents.”

필수 is Sino-Korean (必須) meaning “essential” or “required.”
서류 is Sino-Korean (書類) meaning “documents” or “papers.”

What is the role of 를 in 서류를?