不明白地

"The old lady kept bumbling on about something, but nobody could understand her."

"老太太结结巴巴说个不停,可谁也不明白她说些什么。"

I really fail to understand what actuated you to give up such a promising post so hastily.

我真不明白,什么原因使你这么弃了这样一个很有前途的职位。