"Good morning Calum, it is a good morning."
Translation:Madainn mhath a Chaluim, tha deagh mhadainn ann.
I've also seen/heard a difference between something being sort of "temporarily" or "momentarily" good/bad (because dona and droch work the same way) and innately or emphatically good/bad -- the difference between "a morning that is good" and "a GOOD morning", or "a person that is good" and "a GOOD person", etc.
Maybe it was just coincidence and I'm reading too much into it?
Tha seo glè inntinneach, a Cheabain. I found an academic treatment of this very question at http://argumentum.unideb.hu/2018-anyagok/csonkav1.pdf. The author identifies two factors: abstraction and emphasis. I think deagh charaid means 'a good friend', someone who is good at being a friend, while caraid math is 'a friend who is (morally) good'. But perhaps a native speaker, not a learner like me, should comment.
It’s just the adjective deagh is special as it always works as a prefix of the noun it attributes and always lenites the noun (so deagh mhadainn), and math is a regular adjective that goes after the noun (and is lenited when attributing feminine noun, so: madainn mhath).
As I understand it, tha deagh mhadainn ann means basically there is a good morning in existence, right now, which implies this morning is a good morning, which you express in English as it is a good morning.
The literal meaning is obviously not it is a good morning – in the Gaelic sentence the subject is deagh mhadainn a good morning and the predicate is a dummy ann meaning there, in general existence; in English the subject is a general dummy it, and a good morning is a predicate – but the actual expressed meaning is the same, the equation of general condition around you to a good morning.
Identically to the greeting cited on Akerbeltz wiki tha oidhche mhath ann meaning it’s a good night. Or a discussion on Fòram na Gàidhlig about tha oidhche bhrèagha ann for it is a lovely night, or tha latha brèagha ann it’s a fine day on LearnGaelic.scot.
I have an old dictionary which gives deach/deagh as excellent/very good. In other words, good, with emphasis, a bit like Ceabhain says.
Good morning is a standard greeting to which you could reply the same. However, if it is a nice sunny day, you might regard it as an especially good morning, and say "it is a fine morning!" or "it is an excellent morning" (particularly in Scotland...).
You might find the Tha ... ann discussion helpful: https://forum.duolingo.com/comment/38688604
Here the Gaelic sentence literally states there is a good morning (there, here, right now) and not it is a good morning – but it is used in the same way as a kind of a greeting as English it is a good morning, so that’s how it’s translated.
If you wanted to really equate some it to a good morning you would say ’s e deagh mhadainn a th’ ann (or a th’ innte?) – and perhaps that would also be accepted here. But typically you say just the existential tha deagh mhadainn ann there is a good morning to express the general idea that we have a good morning around today in Gaelic.