"A crab is not a dog."
Translation:게는 개가 아닙니다.
In this sentence, '는' is a postposition which can replace '이/가', the subjective postposition. So '게는' is the subject. But '가' here is a complement postposition, so '개가' is the complement. So, "A dog is not a crab" would be "개는 게가 아닙니다" and "A crab is not a dog" would be "게는 개가 아닙니다".