Questions & Answers about ¿Comiste algo en la feria ayer?
How do we know to use comiste instead of a different past tense form?
Does algo mean "anything" or "something"?
Why is "at the fair" translated as en la feria rather than using a?
The preposition en is used to show location, meaning "in", "on", or "at". We use a (to) when there is movement toward a place (like walking to the fair), but since you are already eating at the fair, en is the correct choice.