i don't think it advisable that Tom be assigned to the job .为什么Tom后的be动词不用was而用原型?

i don't think it advisable that Tom be assigned to the job .为什么Tom后的be动词不用was而用原型?
i don't think it advisable that Tom be assigned to the job .为什么Tom后的be动词不用was而用原型?

i don't think it advisable that Tom be assigned to the job .为什么Tom后的be动词不用was而用原型?
因为有advisable,要用虚拟语气.