Interrogative Adverbs.
Interrogative Adverbs are those which are used in asking questions. They are so called because they are derived from the stems of interrogative pronouns: How, when, where, whither, why.
How can you say such a thing?
When will the monsoon start?
Where did you buy that tie?
Whither away? (= Where are you going?)
Why cry?
Other Interrogative Adverbs are whence, whereby, wherefore, wherewith, whether.