Learn Polish: Goście na bankiecie najedli się lokalnych przysmaków. - The guests at the banquet ate their fill of local delicacies.

The na- prefix combined with the reflexive się expresses doing an action to the point of satisfaction, or even excess. While jedli just means they were eating, najedli się tells us they ate until they were completely full.

Normally you eat an object in the Accusative case. Why is lokalnych przysmaków in the Genitive?

Verbs built with the na- ... się pattern for satisfaction or excess generally require the Genitive case for the object. This is related to the partitive Genitive, which deals with quantities or amounts of something, just like the English translation 'ate their fill of'.

Is goście a regular plural form for gość?

Yes, though it might look a bit unusual. Gość is a masculine personal noun with a soft stem (ending in -ć). These nouns typically take the -e ending for the Nominative plural. Be careful not to confuse it with gości, which is the Genitive and Accusative plural form.

What triggers the spelling change from bankiet to bankiecie?