Recently, a reader asked about the difference between the present perfect tense and the past simple tense in tag questions. Tag questions are short questions seeking confirmation or expressing doubt at the end of a sentence. In "You knew the answer to the question, didn't you?", "didn't you?" is a tag question seeking confirmation that you knew the answer.
"You have bought new furniture, haven't you?" is in the present perfect tense. "You bought new furniture, didn't you?" is in the past simple tense. We use the present perfect for things that happened at an undefined time in the past, things that happened repeatedly in the past, things that happened in the past but in some way affect the present, in questions with "ever" and in statements with "never." Have a look here https://learningenglish.voanews.com/a/everyday-grammar-simple-past-and-present-perfect/2752310.html and watch the video and read the article below the video for some great information. If I visit your house for the first time in a long while, and I see that you have different furniture than I remember, I might say "You have bought new furniture, haven't you?" We use the past simple for things that happened at a specific time in the past. If I see your car in the furniture store parking lot one day and see you or call you on the phone later, I might say "You bought new furniture today, didn't you?" We do not mix the tense of the verb in the statement and the tag question. We would not mix "You bought new furniture,..." with "...haven't you?". The verbs in the statement and the tag question must be in the same tense. Thanks again for the question!