그러나

In a sentence

• 저는 피곤하다. 그러나 일을 해야 한다.

  I am tired. However, I must work.

  Here, '그러나' is used to contrast the two ideas of being tired and the necessity to work.

• 시간이 없었다. 그러나 우리는 준비를 끝냈다.

  We had no time. However, we managed to finish the preparation.

  In this sentence, '그러나' highlights the unexpected success of completing the preparation despite the lack of time.

• 그는 열심히 공부했다. 그러나 시험에 실패했다.

  He studied hard. However, he failed the test.

  '그러나' here indicates a contrast between the effort put into studying and the undesired result of failing the test.