"She is beautiful."
Translation:Tha i bòidheach.
Because the adjective lenites only when it's directly linked to the noun. So 'the woman is beautiful' Tha am boireannach bòidheach. But 'she is the beautiful woman' - Tha i am boireannach bhòidheach. It's only when the adjective is directly modifying the noun (like beautiful woman) that the adjective lenites after feminine nouns.