In a way that breaks a promise — most often the promise between two people who are together. If someone "behaves unfaithfully," they had sex with, or a romance with, someone who is not their husband, wife, or partner.

In a way that breaks a promise用一種違背承諾的方式， most often the promise between two people who are together最常是指兩個在一起的人之間的承諾。 If someone "behaves unfaithfully,"如果有人『behaves unfaithfully』， they had sex with, or a romance with意思是他跟 someone who is not their husband, wife, or partner不是自己丈夫、妻子，或伴侶的人發生了關係，或談了戀愛。

The word is an adverb, so it tells you HOW someone acted: "He acted unfaithfully." Honestly, this is not a common word in daily speech. English speakers almost never say "he behaved unfaithfully" out loud — they say "he cheated on her," or "he was unfaithful." You will meet unfaithfully in writing: newspapers, novels, court reports, church talk, anything careful or old-fashioned.

The word is an adverb這個字是副詞， so it tells you HOW someone acted告訴你某人是『怎麼』做的： "He acted unfaithfully."『He acted unfaithfully.』 Honestly, this is not a common word in daily speech老實說，這在日常口語裡不是常見字。 English speakers almost never say "he behaved unfaithfully" out loud英文母語者幾乎不會把『he behaved unfaithfully』說出口， they say "he cheated on her," or "he was unfaithful."他們會說『he cheated on her』，或『he was unfaithful』。 You will meet unfaithfully in writing你會在書面文字裡遇到unfaithfully： newspapers, novels, court reports, church talk, anything careful or old-fashioned報紙、小說、法庭報告、教會言論，任何講究或老派的文字。

It sounds serious and a little distant, which is sometimes exactly why a writer picks it — it does not sound as angry as "cheated." There is a second, older use, outside of love: to do a job unfaithfully means to do it without keeping your promise to it — a servant, a translator, a person paid to guard something. You will see this mostly in old books.

It sounds serious and a little distant它聽起來嚴肅，還帶點距離感， which is sometimes exactly why a writer picks it這有時候正是作者選它的原因—— it does not sound as angry as "cheated."它不像『cheated』那麼帶怒氣。 There is a second, older use, outside of love還有第二種比較舊的用法，跟愛情無關： to do a job unfaithfully means to do it without keeping your promise to it把一件工作做得unfaithfully，意思是沒有守住對這件事的承諾—— a servant, a translator, a person paid to guard something像僕人、翻譯，或受僱看守東西的人。 You will see this mostly in old books這種用法你大多會在老書裡看到。

Both uses share one idea: a promise was made, and the person did not keep it.

Both uses share one idea這兩種用法有一個共同的想法： a promise was made曾經許下承諾， and the person did not keep it而這個人沒有遵守。