Put the question to the subject of the sentence. I went to the zoo yesterday.

1. Identify the subject: The subject of the sentence is "I".

2. Formulate the question: To ask about the subject, use "Who" or "What". Since "I" refers to a person, "Who" is appropriate.

3. Construct the question: Combine "Who" with the rest of the sentence, adjusting the verb if necessary. The past tense verb "went" becomes "did go" in a question with "Who".

Answer: Who went to the zoo yesterday?