This is the 104th most frequent Chinese word.
当
"当" means "to be", "act as", "appropriate", or "at that time," depending on context.
In '当医生', '当' means 'to become', indicating a desire to assume the role or position of a doctor.
他小时候想当医生。
He wanted to become a doctor when he was a child.
In '当老师讲课的时候', '当' denotes 'when', marking a certain time frame related to the teacher lecturing.
当老师讲课的时候，我们要认真听。
When the teacher is giving a lecture, we should listen carefully.
In '当场的主持人', '当场' uses '当' as part of an idiom meaning 'on the spot', describing a location contextually.