In all of the provided sentences, "fui" translates to "I was." However, the context of each sentence varies:
- "Fui al médico." - "I went to the doctor."
- "Fui a Madrid." - "I went to Madrid."
- "Fui estudiante." - "I was a student."
- "Fui ayer." - "I went yesterday."
The phrase "Fui estudiante." is the one where "fui" means "I was," as it refers to being a student in the past.