그러다

In a sentence

• 그러다가 갑자기 비가 내렸어요.

  And then suddenly it started to rain.

  Here "그러다" links the sequence of events, transitioning to the sudden rain.

• 친구가 늦게 온다고 그래서 기다렸어요.

  My friend said they would be late, so I waited.

  Here "그러다" (in "그래서") shows the reason leading to the action of waiting.

• 그렇게 계속 그러면 안 돼요.

  You shouldn't keep doing that like that.

  In this case, "그러다" refers to continuing an undesired behavior.