• Wh-Words as Subjects in Questions

• If what, who, which, or whose is the subject* in a question, DO NOT use an auxiliary verb.  Use a past simple verb.

  
  

  wh-word (as the subject) + past simple

• Auxiliary Verb (did)	Past Simple Verb
  Who did you invite? (Subject = you)	Who decorated the tree? (Subject = who)

• *Remember! When wh-words are the subject of a sentence, they function as interrogative pronouns.
• 
• Who decorated the tree?

  -You can answer with the verb in the question.

       -Rika decorated the tree.

  -You can answer with was/were or did.

       -It was Rika./Rika did.

  -You can use a short answer.

       -Rika.
•  Click on the play button to hear the audio

• Who baked the cake?  -Jessica baked the cake.	What fell off the table? -A present did.
  You got a lot of gifts. Which was your favorite?  -My favorite gift was the laptop.	This watch is very old.  Whose was it? -My grandfather's.

Source : Alison