"Hello and thank you, Professor!"
Translation:Halò agus tapadh leibh Ollaimh!
You are micing two things I presume:
Leat vs leibh
Whether to use "a" or not.
So, 1. leat vs leibh
tapadh leat - addressing a peer
tapadh leibh - addressing elderly or with respect
tapadh leibh - addressing more than one peer.
And 2. "a" Use "a" in all vocative forms except for the ones that start with a vowel. So while it will be "a phiuthar", "a bhraithar" it will be "tapadh leibh athair" or "tapadh leibh Ollaimh"
It happens in the vocative case, so when you are addressing someone.
Tha màthair agam. > I have a mother.
Halò, a mhàthair! > Hello, mother!
In this example, the word ollamh (professor) is also in the vocative case, and so becomes ollaimh. Nouns beginning with vowels don't get a added to them.
I'm confused - for "Hello and thank you, Professor!" the correct response is 'Halò agus tapadh leibh Ollaimh!' (no comma). I attempted to put a comma in (eg, '..., a Ollaimh!) but this is incorrect.
But in the example you gave above (Halò, a mhàthair!) - there is a comma.
Why are they different? Both are either saying Hello or Thank you, followed by the person you are addressing (mother, professor).
I'm sure I'll get there eventually! EDIT: I've just figured it out - it's the vowel rule (I just needed to read more thoroughly!)
Ewan's question was a year ago and it's likely too late for my answer to be of any help. But in case a later student sees the question and it's having similar problems, I'll point out that I am noticing he wrote what he attempted to put as "…, a ollaimh".
The comma makes no difference to duolingo. It treats all punctuation as spaces, and ignores duplicate spaces so ", _ " (using _ for a space to avoid confusion) gets parsed as " _ _ " which then gets cut down to " _ ". The problem, if you look at ewan's quote, is that he has typed an "a" beforehand that shouldn't be there.
You would say "a mhàthair" to indicate speaking directly to her, but because ollaimh begins with a vowel the "a" is not used here so you just say "tapadh leibh, ollaimh".
I think that's likely what people have been getting wrong
It's called "slenderizing".
I don't really understand it fully but it goes along with lenition a lot of the time. The rules might be somewhere in the notes but I'm not really sure.
Note that Ollamh behind with a vowel and so cannot be lenited, but a different word on the same sentence would be. For example "Tapadh leibh a Sheanair"