1. My wife's having an affair.
我妻子有外遇。
还能这样说：
My wife's secretly dating someone else.
My wife's cheating on me.
谚语：
Have but few friends though many acquaintances.
朋友要少，熟人要多。
2. She is having an affair with her boss.
她和老板有暧昧关系。
还能这样说：
She is intimate with her boss.
She and her boss maintain an uncertain attitude.
3. She often fools around with a married man.
她经常和一有妇之夫鬼混。
还能这样说：
She often runs with a married man.
She often hangs out with a married man.
应用解析：
The earth moves around the sun.
地球围绕太阳转。
4. They had a passionate affair for three years.
他们的婚外恋情持续了3年。
还能这样说：
They had an extramarital affair for three years.
They had a three-year extramarital affair.
应用解析：
passionate的意思有：易激动的；多情的；易怒的，性情暴怒的；热情的，热烈的。
例如：They are passionate about education. 他们热爱教育事业。
5. He admits to an extramarital affair.
他承认了自己曾与别人发生过婚外恋。
还能这样说：
He acknowledges that he has an extramarital affair.
He admits of his mistake of having an affair.
应用解析：
admit defeat 服输，甘拜下风；
admit date 入院日期；
admit impediments 承认障碍；
admitting that 虽说，即使
6. This affair led to their divorce.
这段婚外情导致了他们的离婚。
还能这样说：
They lived apart due to extramarital affair.
Extramarital affair resulted in their divorce.
谚语：
You can lead a horse to water, but you can't make him drink.
师傅领进门，修行在个人。
7. How can you cheat on me, when I have loved you so much?
我是那么爱你，你怎么可以对我不忠？
还能这样说：
As I loved you so much, how can you be unfaithful to me?
How can you have infidelity for me, when I devote so much love to you?
应用解析：
cheat sb. (out) of money 骗某人的钱；
cheat the customs 混过海关；
cheat the journey 消磨旅途的寂寞；
cheat the weary night 消磨长夜
8. It looks like those two are up to something.
最近那两个人怪怪的，关系暧昧。
还能这样说：
They seem to play around with each other.
They look strange and intimate.
应用解析：
be on the up-and up 越来越好，蒸蒸日上；
be up to sb. 是某人的责任和义务；由某人决定；
not be up to much 值不了多少，不太好
9. Immature woman abuses the husband's mistress when he was unfaithful.
不成熟的女人在怀疑丈夫有不忠诚的时候谩骂小三。
还能这样说：
Immature woman would abuse some other woman who has extramarital affair with her husband.
Immature woman hurls invective at the husband's mistress when he has disloyalty.
应用解析：
abuse one's privilege 滥用特权；
abuse a friend 辱骂朋友；
emotional abuse 情绪虐待，冷暴力
10. His wife suspected that he had a mistress.
他妻子怀疑他有情妇。
还能这样说：
His wife had doubt that he had an affair with someone else.
His wife was suspicious of his infidelity.
应用解析：
suspect sb. of sth. 疑心某人干某事；
suspect sb. of a crime 怀疑某人犯罪；
suspect sb.'s motives 怀疑某人的动机