Yes. Tha is the present-tense independent form of the verb bi to be. Also, in this case teine is a masculine noun so the adjective doesn’t change and mòr would be the same in both an teine mòr the big fire and tha an teine mòr the fire is big, but then in feminine, or in plural, where the attributive adjectives change – there would also be a change of the adjective. Compare:
- a’ phàirc mhòr the big park (pàirc is a feminine noun, it causes lenition of its attributive adjective),
- vs tha a’ phàirc mòr the park is big (mòr stays unlenited as it’s now a predicate of the verb tha, not an attribute of the noun pàirc)
- na cait mhòra the big cats – short monosyllable adjectives get ending -a or -e in plural, and adjectives are lenited after short plurals without endings,
- tha na cait mòr the cats are big – here the adjective is the predicate, so it stays unchanged.
In those cases you can distinguish between the adjectives attributing the subject and being a predicate of the verb:
tha a’ bhean mhòr bhochd glic the big sick woman is smart (attributive mhòr bhochd big sick are lenited, while predicative glic smart stays in its base form)