为什么before后面加being given而不是giving

问题描述:

为什么before后面加being given而不是giving
Van Wendel's last three months of life before being given a final,lethal injection by his doctor were filmed and first shown on television last year in the Netherlands.
为什么before后面加being given而不直接是giving?

这里主要是反映了被动式,其次before是介词,后面的动词是要加ing形式,