When you use someone's things, you have to use them with care.
"人の物使う時は大切に使わないといけないよ"
- When you use someone's things, you have to use them with care.
- You have to be careful when you borrow something from someone.
- You have to use the things you borrow with care.
Whenever you borrow other people's belongings. it's important to take very good care of them.
ご質問ありがとうございます。
「人の物使う時は[大切に](https://eikaiwa.dmm.com/uknow/questions/21086/)使わないといけないよ」は英語で「Whenever you borrow other people's belongings. it's important to take very good care of them. 」と言えます。
このシチュエーションで自分のものを友達に貸して、ボロボロになって返ってきた時、上記の英文は適切です。
ご参考になれば幸いです。
アメリカ合衆国 
