"A man is eating a fish and soup."
Translation:Mężczyzna je rybę i zupę.
You can use the word "pan" to refer to an unfamiliar man only when talking to small children.
In grown-up talk, your translation means "You are eating a fish and soup", among few other, even less likely translations.
Yes indeed, 'Pan/Pani' is somewhat similiar to Spanish 'usted', or rather Italian 'Lei/Ella' used in contact with non-familiar people, when using standard 2nd person form could be taken as somewhat inpolite.
"ryba" is the basic, Nominative form, used mostly for the subject of the sentence.
"rybę" is Accusative, used for the direct object. The verb "jeść" (to eat) takes Accusative.