Subject and object questions

In English, questions usually have this structure:

Question word + auxiliary verb + subject + verb + object

For example:

What did you eat for lunch?

Why did they win the match?

What does she usually do at work?

Who did they meet at the party?

However, when the question word is who or what, questions can have a different structure:

Who killed Kennedy?

What eats bananas?

Who lives in this house?

What animal sleeps a lot?

In these questions there is no auxiliary verb and the verb is not in the infinitive form, because the question word (who, what) is the subject of the verb in the question.

Who killed Kennedy? L.H. Oswald killed Kennedy?

What eats bananas? Monkeys eat bananas.

Who lives in this house? My mother lives in this house?

What animal sleeps a lot? Koalas sleep a lot.

As you can see, we can replace the question word with the subject of the answer and we have a correct sentence. So, when the subject of the answer can be used instead of the question word who or what with no change in meaning, who or what are the subject of the verb in the question, and we don't use an auxiliary verb in these questions.

Look at the difference between these questions:

Who cleans the house? My father.

What does my father clean? The house.

In the first case, we can use "My father" instead of "who": My father cleans the house. "My father" / "Who" is the subject.

In the second case we cannot say "The house my father clean", because "The house" / "What" is not the subject. So we use the normal word order in questions and an auxiliary verb.

You can study this lesson and do some exercises in many books. Grammarway 2 is a good option.

And here you can do some interactive exercises.

Exercise one: choose the right questions.

Exercise two: match questions to answers. Discover subject and object.

Exercise three: write questions.