"Next time, please be sure that you have taken all of your belongings from the room before returning the key."
- "Next time, please be sure that you have taken all of your belongings from the room before returning the key."
"next time" 「次回は」
"please be sure that ~" 「〜に確認してください」
"you have taken ~" 「〜を取りました」・「〜をまとめた」
"all of your belongings" 「全ての自分の物」・「荷物」
"from the room" 「部屋から」
"before returning the key" 「鍵を返却する前に」