(+) Cấu trúc: S + used to + V

      My father used to be a doctor in this hospital in 2020.

(?) Cấu trúc: Did + S + use to + V

     Did your father use to be a doctor at this hospital in 2020?

(-) Cấu trúc: S + didn't + use to + V

     My father didn't use to be a doctor in this hospital in 2020.

Cấu trúc thì quá khứ đơn với tobe 

` ( + ) `  : S + was / were + ... 

` ( - ) `  : S + wasn't / weren't + ...

` ( ? ) `  : Was / Were + S + ... ?

` + ` I / he / she / it / chủ ngữ số ít + was ( not ) + ... 

` + ` You / we / they / chủ ngữ số nhiều + were( not ) + ... 

` ( + ) ` : My father was a doctor in this hospital in `  2020 `  

` ( - ) ` : My father wasn't a doctor in this hospital in ` 2020 `  

` ( ? ) `  : Was your father a doctor in this hospital in `  2020 `  ?