Ba is the past tense of the copula? And bhi is the past tense of ta?
Yes. However, ba is also the conditional mood of the copula. And it's more likely to be interpreted as that without context.
So why does Duolingo prefer "Ba ..." ?
Because ba is the past tense of the copula.
After the past copula, we lenite?
Yes, you lenite after ba.
bhi se ina dhoctuir
Anyone know why it is you can't say, 'Bhí sé ina dochtúir' for this question?
There is a difference in meaning between:
Ba dhochtúir é
Bhí sé ina dhochtúir
However the English sentence here could be both, so either is acceptable.
Could you explain the difference? I can't squeeze any difference of connotation from the reference materials I have.
"Ba dhochtúir é" implies that being a doctor was a permanent state of his. "Bhí sé ina dhochtúir" is more temporary; it could mean "He was a doctor (at the time, but wasn't one later)", and it could also mean "He became a doctor".
Because they didn't expect it as an answer. Please report it.
"Bhí sé ina dhochtúir" is also correct, isn't it? Duo doesn't think so.