"She is stupid."
Translation:Tha i gòrach.
Lenition is required for adjectives that directly qualify a feminine noun (i.e. attributive adjectives) - e.g. a stupid girl. Lenition is not required for adjectives that describe a feminine noun as part of a verb phrase (i.e. predicate adjectives) - e.g. a girl is stupid. So we have 'caileag ghòrach', but 'tha caileag gòrach' (or, as here 'tha i gòrach'). It's easier to see the distinction between attributive and predicate adjectives in English, as the verb separates the noun from the adjective with predicate adjectives. There is a similar distinction in agreement for gender with predicate and attributive adjectives in German.
This is covered in the Tips for the Family unit: https://duome.eu/tips/en/gd