This is the 6334th most frequent Korean word.
아니어서
"아니어서" means "because (it is) not" or "since (it is) not."
In this sentence, "아니어서" is used to express the reason for being surprised, forming a cause-and-effect relationship with the preceding clause meaning "not true."
그것은 사실이 아니어서 놀랐어요.
It wasn't true, so I was surprised.
Here, "아니어서" indicates that the lack of rain facilitated the action of going outside, showing a contrasting situation.
우리는 비가 아니어서 밖에 나갈 수 있었어요.
It wasn't raining, so we were able to go outside.
This uses "아니어서" to explain the machine's non-functionality as the reason for the inability to complete the work.