他们是知心好友。用英语怎么说？

他们是知心好友。

They were bosom friends...

they

pron. 他/她/它们，人们，大家，政府，当局，她们；

例句
The storms had abated by the time they rounded Cape Horn.
他们绕过合恩角时，暴风雨已经减弱。

were

be的过去时复数和第二人称单数形式，有时代替 was,用于条件从句、动词 wish 之后等；

例句
Shops, houses, and vehicles were set ablaze.
店铺、房子和车辆全都陷于火海。

bosom

n. 胸部，乳房，衣服的胸襟，家庭般的温暖，胸怀；vt. 怀抱，把…藏在心中；adj. 心爱的，亲密的；

例句
Joan was delighted to welcome her boyfriend into the bosom of her large, close-knit family.
琼十分高兴地欢迎她的男朋友加入她亲密无间的大家庭。

friends

n. 朋友( friend的名词复数 )，资助者，同胞，指做傻事或烦人的事而说话者不认识的人；

例句
Old enemies can become new friends even if all their national interests are not in complete accord.
即使国家利益不尽相同，他们也可以化敌为友。