there is no risk of getting AIDS____(捐赠) when blooddonating.为什么要用ing 形式?是非谓语动词?具体解释下.

问题描述:

there is no risk of getting AIDS____(捐赠) when blood
donating.
为什么要用ing 形式?是非谓语动词?具体解释下.

There is no risk of getting AIDS when (you are) donating blood
省略了东西.when要么加个从句,为了简单,直接加了动名词