Why would you think that you could tell me what to do?
Why do you think you know what's best for me?
ご質問ありがとうございます。
① "I don't understand why you could order me around."＝「なぜあなたが私に命令できると思うのか理解できない。」
② "Why would you think that you could tell me what to do?"＝「なぜあなたが私に何をすればいいかと言えると思うの？」
③ "Why do you think you know what's best for me?"＝「私にとって何が一番良いかを知っていると思うの？」
ご参考までに。
アメリカ合衆国 
