1) "he was not a teacher" "he was never a teacher" "he was not ever a teacher" = all the same 2) "he was once a teacher" "he was formerly a teacher" = the sentences are better and more natural in a positive voice. they have been negated in the examples at (1) 3) "previously he used to be a teacher" 3a) "he wasn't previously a teacher" = is grammatically good, but examples at (1) are already implying that and sound natural. 3b)"previously he wasn't a teacher" = also good grammatically but also the same is implied at examples in (1)

Hi Tam, Jessi is correct and offers good alternatives. However, "He wasn't previously a teacher" would be common use. I'd say "he's never been a teacher", like Jessi offered. Have a good weekend, Stephen

1 is not correct, 2 and 3 are correct! They sound a bit formal, you could also say "he's never been a teacher" or "he was never a teacher."