"My sister has perfect sight."
Translation:Tá amharc foirfe ag mo dheirfiúr.
Why do you use the article in the sentence "She has bad sight / Tá an t-amharc go dona aici", but not in this sentence?
“Sight” is an abstract noun, and abstract nouns used in a general sense take an article. In Tá an t-amharc go dona aici, the presence of the article can be interpreted as “sight” being used in a general sense (“The sight is poor at her”).