“To be a . . . (profession, friend, etc.)”: d - Cchatty

“To be a . . . (profession, friend, etc.)”: differences between shì 是, dāng 当, and zuò 做

Shì 是 is simply the verb “to be”:

She is a doctor.
Tā shì yīsheng.
她是医生。

They are my friends.
Tāmen shì wǒde péngyou.
他们是我的朋友

Dāng 当 is “to be” in the sense of “to serve as,” and is always followed by a certain profession or official position:

She is a doctor at that hospital.
Tā zài nàge yīyuàn dāng yīsheng.
她在那个医院当医生。

He is (serving as) the head of the English Department:
Ta dāng Yīngyǔxì de xìzhǔrèn.
他当英语系的系主任。

Zuò 做 as “to be” is less formal than dāng 当 and is broader in meaning. It can refer to being something other than the holder of a profession or office, such as “to be a parent” or “to be a brother/sister”:

Anyone who is an older brother or sister ought to take of their younger siblings.
Zuò gēge jiějie de yīnggāi zhàogu dìdi mèimei.
做哥哥姐姐的应该照顾弟弟妹妹。