what is an interrogative pronoun?

Interrogative pronoun

Interrogative pronouns are used in asking questions. The pronouns who, what and which are used as interrogative pronouns. 
e.g. Who  telephoned? 
 What  did you say? 
 Which  is your brother? 

a. Direct questions
Interrogative pronouns can be placed at the beginning of a sentence in order to ask a question. Such questions can be referred to as direct questions.

In a direct question, when the interrogative pronoun is the subject  of a verb, the verb follows the subject. In the following examples, the verbs are underlined, and the subjects of the verbs are printed in bold type. 
e.g. What  has happened?
 Who  has been invited?
In these examples, what  is the subject of the verb has happened, and who  is the subject of the verb has been invited. The presence of the interrogative pronoun transforms the statement into a question, and a question mark must be used.

When the interrogative pronoun is the object  of the verb or the object of a preposition, inverted word order must be used, with the first auxiliary preceding the subject of the verb. In the case of verbs in the Simple Present or Simple Past, the auxiliary do  or did  must be used. 
e.g. What do  you  mean?
Which did  she  choose?
What is  he  doing?
To what can  one  attribute  their success? 

In the preceding examples, the subjects you, she, he  and one  are preceded by the auxiliaries do, did, is  and can. In the first three examples, what  and which  are the objects of the verbs. In the fourth example, what  is the object of the preposition to.