ご質問ありがとうございます。
・「I returned the kitten to its foster family.」
(意味)[子猫](https://eikaiwa.dmm.com/uknow/questions/46240/)を里親さんに返しました。
・「I had to return the kitten to its foster family.」
(意味)子猫を里親さんに返さなければなりませんでした。
<例文> Unfortunately, I had to return the kitten to its foster family.
<訳> 残念ながら子猫を里親さんに返さなければなりませんでした。
参考になれば幸いです。
We had a kitten come live with us for a short time, but the other cat we have had for a long time just kept pestering him, so we decided to give the kitten back to its foster family.
この場合、次のような言い方ができますよ。
ーWe had a kitten come live with us for a short time, but the other cat we have had for a long time just kept pestering him, so we decided to give the kitten back to its foster family.
「短期間子猫と住んでいたが、先住猫がしつこくしたので、子猫を里親に返しました」
to give the kitten back to its foster family で「子猫を里親に返す」と言えます。
ご参考まで!