"The beef steak is not good!"
Translation:Friptura de vită nu este bună!
You should look up the declensions of adjectives in Romanian. Basically, "bun"(good) is a four-form adjective. That means it has four different forms depending on the gender and number of the noun:
bun - masculine, singular
bună - feminine, singular
buni - masculine, plural
bune - feminine, plural
Since "friptură" is a feminine, singular noun, we use "bună".
Most adjectives have four forms, but there are such with 3, 2 or even 1.