"Your dog is really friendly."
Translation:Con chó của bạn thật thân thiện.
These lessons seem to be trying to say that "really friendly" and "very friendly" are different. I disagree.
Now, if the sentence were more like "your dog really is friendly" then it would have a meaning more like "actually" and imply that I doubted he was friendly but have just discovered I was wrong. As written though, it's just being used as emphasis and is the same as "very". Rất really should be accepted.