"That is a big house."
Translation:Sin taigh mòr.
Tha sin taigh mòr doesn't work in Gaelic. Gaelic has two "to be"-type verbs. Tha is used to describe things - tha an taigh sin mòr - that house is big - would work. Is (often shortened to 's) is the present tense of the "to be" verb you need to identify things, and has its own sentence structure, which you will come to soon in later lessons - 'S e taigh mòr a th' ann an sin.
Sorry, I do not understand your explanation. In "tha sin taigh mor" we are indeed describing a thing-- a house-- which you say "tha" is used for. The only difference I can see between that sentence, which you tell us is incorrect, and the sentence that you say is correct -- Tha an taigh sin mor-- is "a house" vs. "the house."