"Let's make it in four days!"
I'm partly fluent in Korean, and I've never heard of "나흘", I've always heard of "넷일/사일". If the one I heard more often is the more common term, why teach the one less heard of?
나흘 is the grammatically correct way to count 4 days. 사일 is the lazy way, as my Korean friends tell me.
Can someone come up witj a hypothetical where this sentence may be used?