A law or a decision is retroactive when it does not only begin today — it reaches back and also covers a time in the past. If a new rule is retroactive to 1 January, that past date is treated as the real start, even though the rule was written later.

A law or a decision is retroactive一項法律或決定是retroactive溯及既往的 when it does not only begin today如果它不只從今天開始生效 it reaches back而是往回追溯 and also covers a time in the past也涵蓋過去的一段時間 If a new rule is retroactive to 1 January如果一項新規定溯及既往到1月1日 that past date is treated as the real start那個過去的日期就會被當成真正的起算點 even though the rule was written later儘管這條規定是後來才寫出來的

This is a formal word from law, tax, pay, and school or company rules. People argue about it because it can feel unfair: you followed the old rule, and now a new rule looks back and judges those days.

This is a formal word這是個正式的字 from law, tax, pay, and school or company rules用在法律、稅務、薪資，還有學校或公司的規定裡 People argue about it人們會為此爭論 because it can feel unfair因為這感覺不公平 you followed the old rule你當初遵守的是舊規定 and now a new rule looks back而現在新規定卻回頭 and judges those days去評斷那些日子

Though actually, a retroactive change is not always bad news — a pay rise that is retroactive means you also get the extra money for months you have already worked.

Though actually不過其實 a retroactive change retroactive的改變 is not always bad news不見得都是壞消息 a pay rise that is retroactive一次溯及既往的加薪 means you also get the extra money代表你也能拿到多出來的錢 for months you have already worked補足你已經工作過的那幾個月