"The dog has a family."
Translation:개는 가족이 있습니다.
Normally you could use either with a slight change in emphasis. However, sentences describing possession ("has") are kind of a special case in Korean. The "haver" is always marked with 는 or 은 depending on if it ends with a vowel or consonant, and the thing that the haver has is always marked with 가 or 이 depending on if it ends with a vowel or a consonant. It is a special grammar case, so you just have to memorize that you use it when someone has something. Hope this helps!
You can use both, but it has slightly different meanings. 개는 가족이 있습니다 This sentence could mean that "Dogs have families", as one of the functions of 은/는 (topic marker) is to make general statements. Still learning the basics of Korean, so I might be wrong. Not sure about the meaning for the sentence 개가 가족이 있습니다. Need help from experienced teacher!
The dog is the subject of the sentence. Focusing on the dog has a family instead of the family has a dog. Hence why it was 개는 instead of 개가 (개는 makes the dog the primary subject whereas 개가 makes the dog the secondary subject).
Nope, if for some reason you wanted to say that, you could say 개는 가족이다. A는 B이/가 있다 = A has B. Generally.
But why is 이/가 used instead of 은/는 ? In the sentence "The dog has family". Is family not an object?
는(or 은) is a Subject maker and 이(or 가) is a topic maker. Normally when we use 있다(in this case 있습니다), the subject came first.