do you have to remove the 「です」 to make it natural? or is there other rules? For example, if i wanted to say 「これは友達の犬です」 (this is my friends dog) in a natural/casual way does it become 「これは友達の犬」？ わかりにくいね 😭😭 は 日本語 で何と言いますか？
It is difficult question! In this case, "です" means more polite. But you have to be careful this situation indicates you are showing the dog's picture or something. I think we do not use "これ" for pets. We use "これ" as a subject of pet when we indicate pictures.