"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!
This sentence follows a special structure for saying A has B. The endings determine A as the owner, and B as what is owned.
개는 가적이 있니다 = The dog has a family.
가적은 개가 있니다 = The family has a dog.
--- After you determine who has what, remember also to add the marker based on the word's ending either in a consonant or a vowel ( C / V): ---------- 은 / 는 and 이 / 가.
----- In English grammar, the owner is the subject. We know that is also true for the Korean meaning: In this Korean sentence structure the owner must be placed as the first noun plus it has the 은 / 는 ending.
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!
But why is 이/가 used instead of 은/는 ? In the sentence "The dog has family". Is family not an object?