نیست

In a sentence

• این کتاب نیست.

  This is not a book.

  In this sentence, "نیست" is negating the identity of the subject with its predicate.

• او اینجا نیست.

  He/She is not here.

  Here, "نیست" signifies the absence of the person from the location.

• ما چیزی برای گفتن نداریم،‌ پس صحبت نمی‌کنیم چون نیاز نیست.

  We have nothing to say, so we don't talk because it is not necessary.

  In this context, "نیست" expresses the lack of necessity for the action.