"Her horse eats bread."
Translation:Il suo cavallo mangia il pane.
You always use the definite article with the possessive. The definite article matches its gender to the noun. It's just one of those things Italian does.
il cavallo = the horse
il suo cavallo = his horse or her horse
The only time you can omit the definite article with the possessive is if you are referring to a member of your family. ie "mia madre" - my mother.
Of course, many answers are also accepted without the definite article when such an answer is correct (Il suo cavallo mangia pane is another valid answer) but the purpose of presenting so many of the first sentences with the definite article is to demonstrate that it is in fact more frequently used in Italian than in English.
Most of the time the possessive article goes before the noun, il mio orso, la tua ricetta but on rare occasions if the speaker feels the need to emphasize what belongs to who, then it may go after.
In either case, you can't go wrong if you always stick to keeping the possessive in front of the noun.