"His carrots are not good."
Translation:Morcovii săi nu sunt buni.
The possessive adjective agrees with the possessed noun (here, morcovii). There are four forms for this one: său (masculine and neuter singular), sa (feminine singular), săi (masculine plural), sale (feminine and neuter plural). As the noun morcovii is masculine and plural, the adjective takes the form săi.
Keep in mind that this adjective does not tell you the gender of the possessor (it can mean either "his" or "her"). To make the identity of the possessor clearer, you can replace it with the personal pronoun lui (his) or ei (her).
Why is morcovii lui nu sunt bune wrong? Lui means his right? Is it because its bune and not buni? What is the difference?