What does "My father was a teacher.". The word "was" I wonder if it is strong form . "was" is the past sense so it's maybe emphasised (to express that my father arent a teacher anymore = he's retired now) mean?

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Chào bạn,

I would say that the use of "was" can tell the listener two different things.

The most simple would be that the speaker's father used to be a teacher, and is now either retired or in another profession.

In a more severe case, because "was" indicates past tense, it could also be a subtle way for the speaker to convey that their father was a teacher by profession, but that he is also no longer alive.

Ví dụ / for example (from a film):

Angel: "How do you know sign language?"
Alejandro: "[because of] My daughter."
Angel: "Your daughter is deaf too?"
Alejandro: "She was."