쓸데없는

In a sentence

• 그는 항상 쓸데없는 말을 해서 분위기를 망친다.

  He always ruins the mood by saying unnecessary things.

  In this sentence, '쓸데없는' is used as an adjective to describe '말'(words) to express that the things being said are unnecessary or worthless.

• 그러니까 쓸데없는 소리를 좀 멈춰줄래요?

  Can you please stop saying these unnecessary things?

  Here, '쓸데없는' modifies '소리'(talk/sound) as an attributive adjective to indicate that the talk referred to is irrelevant or needless.

• 이렇게 쓸데없는 일에 시간을 낭비하지 마세요.

  Don't waste your time on such unnecessary matters.

  In this context, '쓸데없는' is qualifying '일'(matters/task), conveying the notion that the matters mentioned lack purpose or importance.