A letter addressed to the previous occupant arrived.
「前の住人」は previous occupant で言えます。
「宛」は addressed to で表現できます。
「中身が新しいクレジットカードだったので、事情をカード会社に連絡したいと思っています」というのは There was a new credit card inside, so I want to contact the relevant company about it のような文で言えます。
参考になれば幸いです。
A letter arrived addressed to the previous occupant.
An important letter arrived for the person who lived here before me.
A new credit card was delivered here for the previous occupant.
「前の住人宛の手紙が届きました」= A letter arrived addressed to the previous occupant. / An important letter arrived for the person who lived here before me. / A new credit card was delivered here for the previous occupant.
ボキャブラリー
letter = 手紙
addressed to = 誰々宛
previous occupant/person who lived here before = 前に住んでた人
important = 重要な、大切な
arrived/was delivered = 届いた
new credit card = 新しいクレジットカード