"Ayer fui a la iglesia."
Translation:Yesterday I went to church.
That's not correct. It's a little confusing because the translation of "fui" says "was/were" as one of the options, but that does not apply to being in places. "Fui" is the past tense of the verb "ir" (to go), which applies in this case, and it is also the past tense of the verb "ser" (to be), which does not apply in this case. The verb "ser" is not used for "being at a certain place".