他们伤势严重。用英语怎么说？

他们伤势严重。

They were badly injured...

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。

badly

adv. 非常，在很大程度上，坏，拙劣地，不利地，有害地；

例句
He was badly concussed, the glass leaving two angry cuts across his forehead.
他遭受了严重的脑震荡，脑门上留下的两道被玻璃划破的口子已经红肿。

injured

adj. 受伤的，受损害的，损伤感情的；v. 损害(injure的过去式)，伤害（名誉、自尊等），（尤指在事故中）伤害，使受伤；

例句
The injured were airlifted to hospital in Prestwick...
伤者被空运送往普雷斯特维克的医院。