To ask where an object is, the construction is a bit different:

          His pen is where? (Where is his pen?)

          The big ball is where? (Where is the big ball?)

          The yellow cat is where? (Where is the yellow cat?)

Notice the modifier follows the object.

To answer these questions, drop the question word and answer with the preposition and the place

          His pen is on the table.

          The big ball is under the car.

          The yellow cat is in front of the house.