I know that both 'Used to' and past simple can be used to talk about things that happened in the past, and that 'Used to' is informal in negative sentences but is the preferred construction when the sentence is positive.
The main function of 'Used to' is expressing a habit that you had in the past, but no longer have in the present. Past simple can be used to the same effect but isn't as clear, as the present situation is unknown.
However, if the sentence already suggests that the situation has changed with past simple, is using 'Used to' still necessary, or even grammatical?
For example, which of the sentences below is correct or more formal?
(Back) When I worked as a teller, I had a decent income.
(Back) When I used to work as a teller, I had a decent income.
I was able to find some examples of the type of construction sentence number 2 uses with advanced book search, but there is an air of tautology around it that is hard to put into words.
In short, which (if not both) sentence is the correct version, and why? Or can both of them be used interchangeably?
- When I went there, I was the top student.
- When I used to go there, I was the top student.