Apply the rules of the language that you are learning. What youre saying is "They dog is black" which isn't grammstically correct and mskes you sound silly lol
Ita alright, this is usually how you form sentences where the subject belongs to someone: "The dog that belongs to them is black" or simply "Their dog is black". Tenses in speech can change between the languages depending on what sounds more natural in that language.
Hi, another native English speaker here (UK this time!). Jack_Mehoff's right 'dog of theirs' and 'dog of them' sounds ridiculous so I'd recommend 'their dog' - much better English. In answer to the original question though 'dog of theirs' is grammatically correct even if it is rarely used but it's different to 'dog of them' which is just wrong.
RE: Their dog, dog of theirs and dog of them
Hola Dan. American native English speaker here. I use the construction that consist of third party possessive followed by a noun (e.g. "their dog") about as often as I use the construction, noun followed by the prepostion "of" and then the third party possessive (e.g. "dog of theirs).
I might say about one neighbor's dog, "their dog is a pretty cool mix breed." And in the same conversation, I might say about another neighbor's animal, "that dog of theirs is a mangy mess since they stop taking it to the groomer.")
However, the construction, noun followed by the prepostion "of" follwed by the pronoun for the third person direct object doesn't since to work in so far as showing possession. Try as I might, "dog of them" doesn't seem to work.