-는 동시에

Shows that two events happen at the very same moment, or that one thing has two qualities or roles at once.

시간·순서Time & sequence

Han: The verb before 동시에 takes 는 without any tense; the past time is already shown by 뛰어나왔다 at the end.

Examples

그 영화는 재미있는 동시에 깊은 메시지를 담고 있다.That film is entertaining and at the same time carries a deep message.

그는 이 회사의 대표인 동시에 실력 있는 엔지니어이다.He is the head of this company and at the same time a skilled engineer.

인터넷은 정보를 쉽게 얻게 해 주는 동시에 잘못된 정보도 빠르게 퍼뜨린다.The internet lets us get information easily, but at the same time it spreads false information quickly.

신호등이 초록불로 바뀌는 동시에 차들이 출발했다.The moment the traffic light turned green, the cars started moving.

V / A with 있다·없다+는 동시에

A+(으)ㄴ 동시에

N+인 동시에

듣다 → 듣는 동시에, 크다 → 큰 동시에, 학생 → 학생인 동시에

Do not put past or future tense before 동시에 (no 았는/었는 동시에); show the tense only on the final verb.
It has two uses: exact simultaneity of two events (often different subjects), and two coexisting qualities or roles (usually with adjectives or N인).
It is formal and typical of writing, news and speeches; in casual talk Koreans usually say -(으)면서 instead, and it is rarely used in commands or suggestions.

Wrong: ~~문을 열었는 동시에 고양이가 뛰어나왔다.~~

Right: 문을 여는 동시에 고양이가 뛰어나왔다.

The verb before 동시에 takes 는 without any tense; the past time is already shown by 뛰어나왔다 at the end.

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