Adding 는 to the negating structure A + 지 않다 stresses that the observation goes against expectations. In speech 지는 is often shortened to 진:
시험을 보는 것이 어렵진 않았어요.
(You might think it was, but) the test wasn’t that difficult.
그렇게 시간이 오래 걸리지는 않을 거예요.
It won’t take a long time (even though it looks like it will).
저는 농구를 잘 하진 않았어요.
Actually I’m not good at basketball (even though I’m a tall American wearing a basketball jersey).
오래되진 않았고 고장 난 적도 없어요.
It’s not that old and it’s never broken down (even though it looks worn out).
여기에서 식당까지 멀지는 않아요.
It’s actually not that far from here to the restaurant (although it looks far on a map).
그의 생일 파티에 사람이 많진 않았어요.
(I thought he was popular but) there weren’t that many people at his birthday party.