"My favorite pasta with ketchup!"
Translation:Мої улюблені макарони з кетчупом!
what is the difference between when you can use мій and using мої at the start of a sentence
Possessive pronouns agree in gender, case, and number with the noun they modify. In this sentence, the noun "макарони" is in the nominative plural, so the corresponding possessive pronoun must be in the nominative plural too, that is "мої."
"Мій" is in the nominative masculine singular (or accusative neuter singular) and would be wrong her.