"This is big."
Translation:Tha seo mòr.
Um. Here's what I think. I am prepared to be shot down by a mod but I'll give it a shot.
"Seo Mòrag" would be "This is Morag", because the "seo" in this case encompasses the "is" part. In that sentence the noun is "Mòrag", so "seo" can act like the verb.
However in the sentence in the question, "seo" is the noun, it is the "this" which is big. It can't be the noun and the verb simultaneously, so you need the "tha" separately. I don't think "Seo mòr" would make any actual sense.
Well, I kinda agree – in that in this is Morag the this is more abstract, just a way of pointing at something or someone close to you; while in this is big it really is a pronoun referring to a material thing (just like it would, but this also carries the closeness), but I wouldn’t say that that’s the reason for the different form used. :)
The grammatical difference is the same as with any other X is Y sentence where you need to choose between the bi verb (tha, chan eil, bha, cha robh, etc.) and the copula (is, bu, etc.).
seo in the meaning this is works as a copula – you use it always when the predicate is a noun or a noun phrase:
- seo Mòrag this is Morag just as in is mise Mòrag I am Morag (Morag is a noun),
- seo m’ athair this is my father (my father is a noun phrase),
- seo cat dubh this is a black cat as in (is e) cat dubh a tha ann it is a black cat (a black cat is a noun phrase).
But if you use an adjective, adverb, or a prepositional phrase, you typically use the bi verb:
- tha seo ann am pàirc this is in a park (in a park is a prepositional phrase),
- tha seo mòr this is big (big is an adjective).
BTW, you can use the copula with adjectives and prepositional phrases too, eg is mòr an cat seo this cat is big, but that adds some poetic emphasis and more feeling of permanency than the regular tha an cat seo mòr (but I don’t think you can use adjectives or prepositional phrases this way after the seo-copula).
Generally in linguistics copula is a verb that connects the subject to its predicate (most often the to be verb, eg. Morag is a Gaelic learner, the cat is black, the teacher turns red).
In the context of Gaelic (and Irish) the term copula has much more restricted meaning.
Copula in Gaelic is the word is in is mise am Pòlannach, ’s e cat a tha ann, etc.
When you want to say X is Y in Gaelic and Y is a noun phrase (you state what or who X is, eg. a Gaelic learner, the Pole, my father, the cat, etc.) you have to use the copula.
If Y is an adjective or a prepositional phrase (you describe what X is like, where it is, what state X is in, etc.) you typically use the bi verb (whose forms tha, bidh, bha, bheil, robh etc. are) – and that verb is not called copula (it is sometimes called the substantive verb, but I think mostly in historical context, regarding Old Irish rather than modern languages).