Hi there,

Which word, an ex-teacher or an old teacher, is used when we talk about a teacher that taught us in the past?

In which case we use ex-noun, and in which case we use old + noun?

Plz help me, thank you very much?
'Old' should, in my opinion, be used to refer to age. 'Former' is much better. 'Ex-' gives me a more negative feeling, as in ex-wife (a split that was not intended when the relationship began).
I would say, "(So-and-so) used to be our teacher."
ex-teacher = a person who used to teach, but no longer teaches
old teacher = a teacher who is advanced in age