"This is coffee."
Scottish Gaelic has two different ‘to be’ verbs which you can’t mix-and-match. There’s a difference between the substantive verb tha which you use to describe things with adjectives and state their whereabouts and modes of being with adverbs and prepositional phrases, vs the copula is which you use to state what or who something or someone is (seo used this way has a copula hidden in there).
Thus this is a dog is stating what this is, you use seo cù. Saying this is big is describing what this is like, thus tha seo mòr.
You can read more about the difference with much more patterns of usage explained and more examples in my Guide to Scottish Gaelic ‘to be’, the linking verbs: https://forum.duolingo.com/comment/45459577