With certain conditions, the quarantine period upon entering Japan has been shortened to three days.
However, for visitors who are sightseeing, this will not apply.
ご質問ありがとうございます。
"With certain conditions,"＝「ある条件付きで」
"the quarantine period upon entering Japan"＝「日本に入国した後の隔離期間」
"has been shortened to three days. "＝「は3日間に短縮されました。」
"However, for visitors who are sightseeing,"＝「しかし、観光者は」
"this will not apply."＝「対象外です。」
ご参考に。
アメリカ合衆国 
