"Polish is an important language, but it is not more important than Irish."
Translation:Is teanga thábhachtach í an Pholainnis, ach níl sí níos tábhachtaí ná an Ghaeilge.
Well, the adjective is the comparative form of "tábhachtach" (important) and since "tábhachtach" ends with -ach, the ending -aí is added in place of -ach. So "tábhachtach" becomes "níos tábhachtaí" (more important) (the superlative also uses tábhachtaí: is tábhachtaí (most important)). There is a full explanation of this in the notes of the Comparison skill :)
From a prior lesson's notes: The copula is for when you're introducing something or someone, like "Is bean í" (She is a woman) or "Is úll é" (It is an apple). Bí is for when you're describing something or someone, like "Tá sí ard" (She is tall) or "Tá sé blasta" (It is tasty).
Why then is the copula used here since we are "describing" Polish as important?
The copula is used to classify nouns e.g. is mamach é an cat. to classify the cat as a mammal. In the sentence given in this exercise Polish is being classified as an important language, so the copula is used. To describe Polish as important one would say Tá an Pholainnis tábhachtach.
In general when the predicate of an English sentence is a noun phrase the copula is the appropriate way to translate the sentence into Irish. Tá should only be used for adjectives, prepositional phrases and progressive structures.