Subject and Object Questions

Object Questions are partial questions in which the unknown information is an object. These are the questions that we have dealt with up to now, and depending on which is that unknown information we will use an interrogative pronoun or another. 

The rest of the sentence works as any interrogative sentence, with auxiliary do or inversion subject-verb.

- Where did you find her? She was hidden under the bed.

- What time is it? Three o'clock.

Subject Questions are also partial questions, but the unknown information is the subject, not an object. Thus, the interrogative pronoun is the subject of the interrogative sentence and it will not allow any of the changes for the questions, so you can not use the auxiliary verb do or make the inversion subject-verb.

This way, the Subject Question will keep the structure of any affirmative sentence with the verb in 3rd person singular or plural.

   - Who is the captain of the team? My brother is the captain.

   - What chases rats? Cats chase rats.

The only interrogative pronouns that allow both types of questions are Who, What y Which, the rest only admit object questions.